I don't have a date for you but as I understand Kimber of Oregon they started serial numbers over with every caliber. You could then reserve your number for every caliber that they came out with. So you could have a .22LR, .22Mag, .22Hornet, .22K Hornet, .218 Bee, .25/20 all with the same serial number in the M82. I believe that the M84 was also numbered the same way. Calibers, that I know of, for the M84 were. .17Rem, .222, .222Mag, .223, 6X45, 6X47, 250 Savage, .35 Rem, and a whole lot more that I probably forgot.