Y had nothing to do with how many were made. The ATF 1968 Gun Control Act required that an alpha letter be at the beginning of the serial number. There was pre-64 forged steel machined receivers used up to and after 1968. The cast receiver guns were cataloged in 1972, and not in the 1973 catalog. They skipped several serial numbers when they re-introduced as a production gun (investment cast receivers and other parts). No one calls the other Winchester models by the prefix number just the M12's. Model 21's had a W' model 70's had a G and so on with other guns. The model 12 (New Haven made) was never discontinued until they ended it again as a production gun. It was moved ( approx. 500 pre-64 parts and receivers) to the Custom Shop as a high $$$ special order gun like they did with the M21's in 1959. The biggest problem issue with the new production guns was the screw at the muzzle end to retain the vent rib. It was a bad design and should have been pinned. The gap between the pre-64 guns and the re-introduction as a production gun was the death to its popularity as shooters went to other guns such as the Perazzi TM-1. Not very many even choose a pump gun today.