When the A74 between J12 and the border was being upgraded to motorway in the 1990s it was always intended to have one single number for the whole motorway and it was assumed that it would eventually become the M6 when all the A74 was replaced by motorway.
Because the order of construction meant there were some sections of motorway between non-motorway parts of the A74, A74(M) was used as a temporary number for these isolated bits, since they didn’t join with the rest of the M74.
Many of the signs on, or directing to, the motorway which say “A74(M)” are covering up “M6” - the intention was that these would be uncovered once the construction was complete. And the DVSA site ended up with an M6 sign which never had A74(M) stuck on top.
By the time the Highways Agency eventually got round to completing the motorway on the English side of the border, nine years later, any ideas to renumber the Scottish side had long been forgotten - and the by now devolved Scottish Government, were progressing with the extension at the Glasgow end under the title of “M74 Completion”.
So we’re now stuck with what was supposed to be a temporary number for half of the Glasgow to Gretna motorway.
