Strictly addressing the CZ 75, it?s a change in the numbering sequence over time and production numbers. The original serial scheme was just numbers in sequence that ran into six digits by 1987. It changed to ?A...Z? plus 4-digits in 1988 until that ran out sometime in 1994. They flipped the sequence to 4-digits plus ?A....Z?. That ended around 1999 so along came ?AA? plus 3-digits. I think that sequence ended around 2003....not sure where in that sequence it ended. Someone with guns from that period can fill in the gap.
Since around 2004, the current sequence of ?A....C? plus 6-digits has been in effect across all models. The .40 S&W pistols had a different scheme until around 2004. The USA produced P-10s now have their own unique serial numbers.
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