Well done
@Tribansman, I thought there was a fair chance someone would recognise it. If not, I was going to help with a couple more pictures from the same time which combined would definitely have put you on the right road:
In pre by-pass days Gatehouse used to be transformed at certain times of the day by a procession of vehicles of this type, quite an exciting feature when on holiday but no doubt the townspeople were glad to see the back of them.
When I passed through in 1984 I'd travelled up overnight on the train to Dumfries and rode to Stranraer for a lunchtime ferry, all the way on the A75 apart from a little corner-cut east of Gatehouse. I don't remember it being anything other than delightful.
Over to
@Tribansman ...