A Readers Digest/AA Atlas undated but, from 67/68 based on various new roads, has the A38 running down to Torpoint Ferry. The A374 goes over the Tamar bridge and on to Marsh Mills, where it meets the A38 coming out of Plymouth.
A 1962 OS map has the same, whilst a 1971 OS has the present arrangement, with A374 down to Torpoint, plus the Plympton bypass is open.
Wiki mentions none of this so, is the Digest wrong, or did it take a few years to swap numbers around?

So they weren't swapped with the opening of the Tamar Bridge? I was sure that opened as the A38.

I've just found a date on the inside back page of the RD/AA Atlas of 1966 so, the mystery deepens. I wouldn't have thought the AA would get that wrong by 5 years.

Just looked at my 1967 edition... A38 via the bridge.

It appears from a notice on this page of the London Gazette that the trunk road was re-routed in 1963 but that the old road numbers were still in effect at that time.

The bridge itself is of course non-trunk, and the then A38 (today's B3413) through Plymouth may well have been non-trunk in those days, being inside a county borough.

Edit: the 1968 OS Route Planning Map shows the A38 number and the primary route following the new route via the bridge.

I have an AA map from about 1965 but can't find it at present.

Whats interesting is the article refers to the London Penzance Trunk Road. I thought this had always been A30 -A303 - A30 and the A38 was something different.

Though the Basingstoke-Exeter section was rerouted in 1958 from the A30 to the A303, the London-Penzance trunk road has followed the A38 between Exeter and Bodmin ever since it was created in 1936 (see page 24 of the 1936 Act).

The A30 from Exeter to Bodmin via Launceston didn't become a trunk road until the second and last major round of trunk road creation, which was in 1946 (see page 19, or 21 by the PDF count, of the 1946 Act).

The 1966 RD/AA atlas must have been wrong then about A38. It would have come out in 67 and, shows M4 Newport as open, which was in May 67 according to the Motorway Database. However, another probable mistake, is that the M4 ends at M32 and, is a dotted line to A46 Tormarton, with nothing beyond there. M32 goes to J1 only.

Tormarton to Almondsbury opened in Jan 66. M32 to J1 in Sept 66, along with all of Almondsbury, except M5 northwards. I wonder who would have used the Jan 66 stretch then? M4 J19 and J20 wouldn't have gone anywhere, so all you could do was to drive from Tormarton to Almondsbury, cross the barrier-less reservation and come back again. Is Jan 66 correct? Sept 66 makes more sense.

Perhaps somebody could enlighten me on something that has had me puzzled since I was about 12 years old (late 1960's). The Tamar Bridge was open by then but the A38 (which we followed) was the ferry route. On crossing into Cornwall I was most confused by the apparent interchange of the A374 and A387 between the road signs and the map, which quite upset my navigation!

From the point where these two roads meet, which one goes to Torpoint, and which one goes to Looe and (the place we were trying to get to) Polperro? Had they been recently swapped?

As far as I know the following is consistent with everything in this thread and the maps I've looked at:

The A38 has always gone to Liskeard and Bodmin.

The A387 has always started at the junction south of Trerulefoot and west of Polbathic, and gone to Looe and Polperro.

Originally the A38 ran via the Torpoint ferry, while the A374 ran via the Saltash ferry to Trerulefoot. From St Budeaux, across the ferry and through Saltash, the A374 was the minor partner in a multiplex with the A388.

The trunk road followed the A38 but was suspended through the county borough and restarted at Torpoint.

The Tamar road bridge opened in 1961 but neither the road numbers nor the trunk route were adjusted promptly.

The trunk route was routed via Saltash in 1963 (restarting there instead of Torpoint) but the road numbers were still not changed.

The road numbers were changed about 1967, and on the Cornwall side have not changed since. The A38 swapped with the A374 east of Trerulefoot.

Through Plymouth the A38 has followed three routes: first through the city centre, then along what is now the B3413, then the present route.

No, not always. It originally started in Plymouth. Bodmin to Plymouth via the Torpoint Ferry, including the section between the ferry and city centre was the A389.

Again, not always just previously. Originally it ran from Saltash to Trerulefoot. The road to Looe was the B3250 with the B3253 running Liskeard to Polperro via what is now the B3254/A387. Trerulefoot to Looe (but not Polperro) became an extension of the A387 by 1930 according to the historical mapping on SABRE.

...again previouslyOriginally the A388 started in Saltash with the A387 crossing the ferry at Saltash and running into Plymouth. Interestingly the A388 ran through the town centre and the A387 ran around the north of the town centre on the present day B3271. The road between Forder Valley and St Beaudeax - now the B3413, before that the A38 - started out as the B3246. It had changed to the A3078 by 1932. The whole route from Forder Valley to Trerulefoot via Saltash had become the A374 (originally used in East Devon) by 1946. The A389 via Torpoint to Bodmin had also changed to the A38 by this time.

Thanks for the corrections on the earlier history.

Some roads seem to have had 5 different numbers.

In the 1950 AA Road Book, both the town plan of Plymouth and the main atlas map show the A388 from the city centre to St Budeaux, and the A374 round the north of Plymouth. The A388 is shown as the visible face of the multiplex of the two, along part of Wolseley Road to the ferry and along New Road, Saltash.