In the early mac ROM (128k and earlier, which displayed 6 hex digits for sadmacs instead of the two rows of 8 digits like later macs), the first 2 characters indicate the test being performed. The second 4 characters are information about the test being performed. For those following along at home, for 128k ROMS with checksum 4D1F8172, address 0x15C is where the printing of the sadmac error code happens. The bottom byte of register A4 contains the test being performed (1st 2 digits) and the bottom word of register D1 contains the last 4 digits.

Test 1 is the ROM test (stored in A4), this starts at location 0xD76 of the same ROM. Register D1 is used for computing the checksum. If the checksum is computed correctly, it moves to the next test, incrementing A4.

In summary, this appears to be what the Apple docs say: ROM failure. The lower 4 characters really are "insignificant" as they are just book keeping data from the checksum calculation. My suggestion would be to try replacing the ROMs with ones from a known good unit. I have personally used 27C512 EPROMs in place of the stock ROM chips. De-interleaving your ROM image into the 2 separate high and low ROM chips would be an exercise left to the reader.