I was thinking the 948 was designating Sept/1948.
I don't think anyone at the time or for the next 100yrs would think by looking at the gun that it was proofed in 2048 instead of 1948.
The other Birmingham Proof Date Codes only used a Letter to designate the yr. Nothing to tell you it was 19xx or 20xx.
The style of the Birmingham Proof Date Code used will tell you what yrs it was in use/ under what Proof Law in effect at the time.
The yrs the code lapsed in general use was 42 to 49. No real need to tell anyone it was '19' 42 to '19' 49.
The M ?,,,,maybe a Proof Inspector/Examiner like on the other Birmingham Proof Date Code marks.
