>>64487381
upgrades were based on military requirements and designations, M9A1 adds a rail, the M9A2 was a proposed change that never happened, the M9A3 was an attempt by Beretta to keep the M9 contract when they started talking about the MHS program - the M9A3 was offered with most of the features they wanted AND at a lower cost per unit than the existing M9 contract. The M9A4 was an update of the M9A3, I don't know if it was offered to the military other than "we still exist, it's not too late to go back".
The 92 does have an series of upgrades
92 Vertec (moderization, swappable backstraps, dovetail sight, rail mount)
92X (refresh of vertec)
92X RDO (refresh of vertec)
92Xi (SAO frame lever)
92GTS (DA/SA frame decocker lever)
split off
90-Two, partner project with the PX4 I think where it shares styling and changes the frame to not be as easily cracked by .40 S&W by adding a soft aluminum recoil buffer. Has a PX4 style decocker/safety which then would evolve into the M9A3 style decocker.
92A1, refresh of the 90-Two with less style