All CCs that I have seen begin their 'serial' numbers with SG, but that's not to rule out that more than one logic board manufacturer could have been involved. That is certainly the case in other Mac models. There are differences in colour (tan or green) in CC MLBs, and closer attention may reveal other differences than those of the CUDA switch and battery type.

I misinterpreted you as having written that the early one carried the 4.5V batt., and it therefore appears that use of 4.5V was a middlish period occurrence. A strange departure from the norm that was in all Compact AIOs after the initial use of cylindrical 4.5V batteries in Macintosh to Plus.

de