The books I have don't go back that far so it could be a 1961. R would be 1961 but I haven't seen the M before nor have I seen the serial number on the ID pad. But that is an early block casting so it does make sense that it is a 1961 engine.
No, not if you know these early engines, then it makes sense you don’t know what you are typing. I know you wrote a book, but not on early year 413 block stampings.
A 1961 will have R41 with the 3 missing. Then the month and day like on a 440 block.
15 years in the 300 club taught me these things.
