I've noticed something about this 3 digit code. If the first digit is 1, then I notice on some of the smaller chips, I see 91. I'm fairly sure this means 1991. If the first digit is 2, then I see 92 on some of the smaller chips. If the first digit is 3, then I see 93 on some of the smaller chips.

So, the first digit represent the year of manufacture:

1 = 1991
2 = 1992
3 = 1993

I've never seen a SCC-1 with the number 4. Also, in 1994, the MKII / SCC-1A/B was released.

Edit: now to try and figure out what the remaining 2 alphanumeric digits mean...

It is an MPU-401 on a chip (RJA-401-HB).
It contains the CPU, SRAM and logic array in a single package.
The chip on my SCC-1A is marked as 4B1.