Page 244 of The Lee-Enfiield... By Ian Skennerton says that some BSA No4's had CC as a prefix.
"However, some CA and CC prefix numbers have been noticed more recently so production extended beyond War Office allocations."
Fazakerley ROF used the A suffix from 48-57 but used the PF. From 44-47 FE, no letter prefix, then A-Z, then AA-AF, all with 4 numbers. I'm wondering if your No4 was a BSA Mk1 converted to a Mk1/2 at Fazakerley during the retrofitting post war to make it like the MkII's.
I just found reference to the A suffix being used by ordnance depot or armourer would designate a rifle for handfitting because of some variation in the action body, by stamping an A after the original serial number. I'm really wanting to know if you actually have a No4 Mk1/2. Pictures or as much info as you can provide would help greatly.
As for the MAS, it might just be an inspection stamp as I could find nothing about that stamping.