Hey Al,
From what I've seen of your work and research, I figure if you can't find it or don't know it, nobody does !
British and Canadian markings have always been a pain in the arse to figure out !!
We Colonials supposedly followed the British style but there always seems to been exceptions to the rules !!!
The black and white photos taken at the time invariably don't help because the tones of the red, green and blue paint used seem to look about the same.
When Ultracast started producing their Canadian Sherman decals, on some of the sets they ended up giving you a choice of Arm of Service markings.
Either white numerals on a red square like an armoured regiment's AoS or white numerals on a 1/2 green 1/2 blue square, just like a armoured recce regiment or armoured car regiment's AoS.
The M4A2 Sherman and Ram II gate-guards at the local British Columbia Regiment's Armoury have a white 53 on a red square.
Ultracast's BCR markings give you a white 53 on a green/blue square.
Like I said before, it's a pain in the arse and can drive you nuts !!!
PM any info you get if you ever get it sorted out.
Thanks and Cheers
John