Firstly, thank you Simon. This whole discussion has got me wandering down research rabbit holes which I'd almost forgotten just how enjoyable they can be!
So, I went looking for your "smoking gun" document - the one that would give you that comfy warm feeling. There wasn't one - but there is lots of different documents for which the story can be put together, and the final clue related to a completely unrelated road to the Morriston Bypass in all but number - the A48(M) Llantrisant Radial; though it also takes in some other questions like, "why is the A48 Cowbridge Bypass so large?". I also discovered in my research references to the A48(M) Pyle Bypass, which was new to me, so I dug further.
The story (so far) of the numbering confusion starts in July 1967, with the publication of
Wales: The Way Ahead. This Command Paper was written all about the future of Wales, not just about the road network - but clearly Communications is a large part of it.
Wales: The Way Ahead wrote:
An example of one new road that is required is one from the present termination of the M4 Motorway to the Bridgend area, referred to in paragraph 53(b) below. This would serve a useful purpose in providing an outer by-pass to Cardiff, in relieving congestion on the A48 road between Cardiff and Bridgend, and in speeding up communications with West South Wales.
(a) providing dual carriageways along most of the A48/A40 road as far west as St. Clears;
(b) a new road from the existing end of the M4 Motorway to the Bridgend area, providing an outer by-pass to Cardiff
So what's clear from those two extracts (which matches documents in the National Archives that say or suggest similar) is that the M4 west of Newport wasn't fully planned in July 1967. Instead, what was planned was an upgrade to the A48 to Carmarthen, and then the A40 west to St. Clears. This can also be seen in the below map extract from the document, where it's clear that improvement to the A48 was the preferred option at that time (hence the Cowbridge Bypass):
So, I jumped back a step, and took a look at the Welsh roads programme for 1967.
Cymru-Wales 1967 wrote:
In South Wales the project which will do most to open up for new industrial development the area of the South Wales coalfield most affected by the programme of pit closures is the extension through mid Glamorgan of the M.4 from its present terminal at Tredegar Park, west of Newport. Between Tredegar Park and Swansea and the West the greatest obstacle remaining to the smooth passage of traffic is the congested centre of Cardiff. A new road built to motorway standards, by-passing the city to the north, passing near the developing areas of Llantrisant and Bridgend and eventually linking with the Pyle By-pass (which is already included in the " Preparation Pool ") and the existing Port Talbot Motorway By-pass would provide a new spine to strengthen and sustain the changing industrial life of the most densely-populated area of the Principality.
The line of a new road through mid Glamorgan which would serve this purpose has for some time been shown in the County Development Plan. In view of its undoubted importance in the future the Secretary of State announced in the Welsh Day Debate on 30th November that the Government had decided to assume responsibility for its protection. In the meantime work would start as soon as practicable on the preparation of a scheme for a new dual carriageway road from the Gabalfa Interchange (the western terminal of Eastern Avenue) by-passing West Cardiff and serving the mouths of the Glamorgan Valleys. Part of this road from south-east of Llantrisant towards Bridgend would eventually be incorporated into the M.4 extension.
This is the first mention of the M4 motorway extending west of Newport, and talks about protecting the line of the road through Glamorgan before meeting the Port Talbot Bypass. There's no mention of the motorway going further west than Port Talbot, but included in the Road Programme itself, within the "Trunk Roads" section is indeed the
A48 Morriston By-Pass (and the
A48 Pyle By-Pass). Note that as neither are listed within the "Motorways" section (which is just the sections of the M4 from the Severn Bridge to the end of the Newport Bypass), that it can be surmised with high probability that at that point the Morriston Bypass was indeed proposed as an all-purpose road.
So, let's step forward a year into looking at the Welsh road programme in 1968.
And here we have it - the Morriston Bypass proposal has now been upgraded to motorway (as has the Pontardulais Bypass), but unlike the sections to the east, they're both clearly numbered as A48 but still as motorway. This format isn't uncommon in reports from the time to see as a shorthand because it's part of the A48 Trunk Road, so in this case for A48(M); which is exactly in line with how the Ax(M) numbering was designed to work from the outset.
What can also be seen is that the Morriston Bypass is a higher priority to all the others, as it's the only one in the firm programme. Interestingly, the A48 Pyle Bypass is still in there further down in the table, just listed as an all-purpose road still.
Within the body of the document, it confirms the motorway status of the Morriston Bypass
Cymru-Wales 1968 wrote:
London-Fishguard Trunk Road (A.48/A.40)
[...]
Proposals for the line of the Morriston By-pass, which is to be built as a motorway, were published on 24th May. Objections were still being examined at the end of the year.
Then further down, there's a really interesting map.
This clearly show that there was an intermediate stage in the proposals for the M4 through Glamorgan, where traffic would use Eastern Avenue and the Llantrisant Radial to pass through Cardiff, and the A473 would be widened to meet the A48 at Bridgend; with what we know today as the M4 in the middle. This also explains why the M4 was constructed in the order it was.
So, let's jump ahead another year to the 1969 road programme:
Cymru-Wales 1969 wrote:
In South Wales further substantial progress was made in implementing the strategy for improving communications along the line of the M.4/A.48 road; in April the Secretary of State announced that he had set a target date of 1972-73 for a start of work on the Cardiff Intermediate By-Pass from the north of Cardiff to Bridgend.
London-Fishguard Trunk Road (A. 48/A .40)
[...]
Statutory Orders were made fixing the line of the Morriston By-pass Motorway which will extend from the Lonlas Roundabout on A.48 to Llangyfelach By-pass. A contract for £385,249 was let in September for advance works preparatory to the letting of the main contract in 1970.
[...]
Preparatory work continued on the following schemes [...] Cardiff Intermediate By-pass, which will provide a continuous length of dual carriageway road to motorway or near motorway standard from the Gabalfa Interchange in Cardiff to the Waterton Roundabout on A.48 at Bridgend; the extension of the M.4 motorway from Tredegar Park to St. Mellons; the Pyle By-pass (motorway); the Pontardulais By-pass (motorway)
Note that the Pyle Bypass is now also listed as a motorway section of the A48, hence A48(M), and that there's still no real firm concept of the M4 west of Bridgend.
Then in 1970, everything changes.
Cymru-Wales 1970 wrote:
Further west, work started on the Morriston By-pass which is part of the M4 Motorway.
That's a bit sudden! So, let's see what is said further on in the document.
Cymru-Wales 1970 (published June 1971) wrote:
Extension of M4 Motorway
The Government re-affirmed their intention to provide a dual carriageway road to motorway or near motorway standard from the Western terminal of the M4 Motorway at Tredegar Park, Newport, to the Swansea area by 1976. It is hoped by this date to have completed the improvement to Pont Abraham, beyond Pontardulais.
In August a contract was let for the construction of the 4 mile-long Morriston By-pass which will form part of the M4 Motorway. Preparatory work continued on schemes to provide further sections of the M4 between Tredegar Park and St. Mellons, Llanilltern and Pencoed, Bridgend Northern By-Pass, Pyle By-pass and Pontardulais By-pass. The total length of these schemes is almost 41 miles.
On 24 April, 1970 a draft order under the Highways Act 1959 was published proposing the "River Route" for the Llantrisant Radial Motorway from Gabalfa, Cardiff to Llanilltern, Glamorgan.
In April 1971, the Government announced the addition to the Trunk Road Preparation Pool of the M4 Cardiff Outer By-Pass running from St. Mellons east of Cardiff to Capel Llanilltern, Glamorgan. The question of how the Llantrisant Radial Road would fit in with the Planning of West Cardiff is to be discussed with the Cardiff City Council.
Constructional work continued throughout the year on the three-level interchange between Western Avenue and the A470 Cardiff-Merthyr road at Gabalfa, Cardiff. In February, a contract was let for Stage 2 of the Cardiff Inner By-pass (Eastern Avenue) between St. Mellons and Gabalfa. Work is expected to take about two years to complete.
And here we have it, and all the dates tie up completely with those elsewhere - in April 1971, the plan became to extend the M4 to its present western terminus along its current route; which is why from before that date all references to the Morriston Bypass (and the others...) are A48/A48(M), and after that date is M4.