The first letter, "Z" represent 0 and is first number in serial#, the next letter and first number represent month/year of manufacture. the last numbers are the rest of the serial number.

<First Character in serial number>, <2 character date mfg code>, <reset of serial number, usually 4 digits>.

Z=0, A=1, B=2 etc.

The 2 character date of mfg code is like "02" is Jan 1981. "01" is Dec 1980. it reset in Nov 1980 = "00" and in 1988 or 1989 as "00" again.

So starting with ZA or ZB, nothing to do with bios version, but if you had like Z**0002, sure, that might mean v1.00 if they never updated it..

The "clipping" behavior relates to an internal volume setting, and is not remedied by a ROM upgrade. For most cases, the solution is to simply reduce the Master Volume, via SysEx, prior to gameplay.

The "clicking" behavior - poorly understood, if at all, by many who mention it - is addressed by a ROM upgrade. It's also something of a nothingburger. Are you planning to play either Steam-Heart's for the PC-98xx, or Ultima IV for the Atari ST using my SysEx configuration? It's unlikely that you'll ever encounter this behavior with a unit having the v1.00 ROM otherwise.

In any event:

< Ver. 1.02 >
CM-64 --- ZB23350 and up
CM-32L -- ZB24600 and up
DM-48A -- ZB20600 and up (CM-32L Atari bundle)

Thanks a lot for the amazing info people! It's really appreciated! I have made an error - I have misinterpreted a video on youtube that mentions the ROM 1.00 issue, and now that I've read the answers above, I realise there is nothing to worry about. The intended purpose of the unit is for PC DOS gaming.

BTW, my serial numbers start with -

ZA41
ZB14