I can’t answer your question, but thought this photo may be of interest. I was with LAD REME, 18/26 Sqn RCT from 1972 to 1974, serving for a year in Lisburn and a year in Ballykelly, where the photo was taken. We had 2 or 3 coaches there and a few more in Lisburn, all painted in different colours, but all army coaches, not civilian.
Apologies for poor quality of picture.
Steve.