AIUI, parts of the current carriage way were the original hard-shoulder of the A74 which used a highly abrasive surface.
hm, the A74 from what I vaugley remember was a narrow dual carriageway and a wide single carriageway, also with hard shoulders on motorways being a radical idea in Scotland (see the M90) at the time I'd be surprised if there was hard shoulder at all on the dual carriageway sections. I do remember a rather large parking area at the summit and sitting in it for a bit to get a rest from the long queue crawling south, or probably so the parents could get a rest from me and my brother moaning by feeding us.
My recollection was that on the dualed sections one carriageway was converted to cycle track and then the edges filled in (or not as the case is near moffat where it's hidden in the trees and utterly useless despite being full width due to the amount of debris) .
However what I have read is that when it comes to the A74(M) the contractor was given the option by the then secretary of state for Scotland to build either a D2M with the ablity to be widened to D3M at their cost in future or to be built as D3M; the contractor suggested that it could be done by building the 3rd lane to Hard Shoulder standards due to the fact that heavy vehicles are banned from it. The result of that can be seen in the surface in places.
https://www.sabre-roads.org.uk/wiki/index.php?title=A74Does say some section had "1m hard strips"
https://www.sabre-roads.org.uk/wiki/index.php?title=B7076Describes what each section is formed out of; so it's a mix of the pre-1960s upgrade, the 1960s upgrade and new sections. And it's all a mess!
