Hello,
I've only just found this website, and, by chance came across this chat. I don't think I can add any real info to help with it, other than the fact that my dad was a Don-R with the Royal Army "Sleeping" Corps (There were, according to him, other, less polite terms used for the "S"). Anyway, dad's name was AVH "Vic" MASON. He rode (the DR's most popular m/cycle) a Matchless. Was evacuated, along with thousands of others, from Dunkerque, and was in Nth Africa, but never got as far as Germany.
He died in 1983 (63 years old), and, although he told me many of his exploits, I wish I'd been "grilling" him for more at the time.
I hope soon to post a couple of photos of him in DR battle-dress, on the "Matchy".
This post won't have helped you much, but just clears the fact that there were, at least, two RASC Don-R's named MASON. Good Luck with your search.