That might have been describing something like the "London - Carlisle - Glasgow - Inverness Trunk Road", which certainly did exist as a single entity. In fact, there was a rejected proposal in 1934 to extend the A6 over the A74 and A82 (ie: the rest of the trunk route) to terminate in Inverness.
IIRC I once read that, had Southern Ireland not become a separate country before the road classification system was introduced, then the A6 number might have been given to the London-Carlisle-Glasgow-Inverness route in its entirety, with the A1 following the A9 route to Inverness (can't recall whether the plan was for it to go all the way to Thurso though). However, equally I remember reading another suggestion that the 9-zone may have reserved for Ireland, with the 8-zone covering the whole of Scotland north of the A8, in which case the A6 and A1 would have had to terminate at Carlisle and Edinburgh.
If Ireland hadn't split (just a hypothetical scenario, rather than a political point) then personally I'd have made the actual 8-zone the 7-zone, with the actual 9-zone becoming the 8-zone, and the A6 continuing to Glasgow with the 5-zone to the west and the 6-zone to the east.