I think I must have been mis-remembering what was at Canuck, so here's a list of prefixes to regimental numbers and the corresponding Military Districts (c. 1939). This old Canuck page shows where the districts were located:
http://www.canadiansoldiers.com/orga.../districts.htm
Code:
A MD1
B MD2
C MD3
D MD4
E MD5
F MD6
G MD7
H MD10
K MD11
L MD12
M MD13
P prefixes were for Permanent Force members who went active, W for CWAC's. Some U and X nnumbers were assigned overseas.