I don't know about other manufacturers, but Marlin made some Model 62s without serial numbers. There was a voluntary recall where, if you owned a 62 without a SN, you could send your rifle back to Marlin and they would stamp the receiver with a serial number. I've seen both varieties and have a 62 in 30 Carbine that was returned and hand-stamped with a serial number. I have the invoice from Marlin that was returned to the owner with the newly stamped rifle.