What type of ammo is used in a Winchester model 12 dated 1917 12 gauge?

modern ammunition is acceptable. However be careful that you do not use any of the 3" magnum cartridges if the barrel is marked 2 3/4. When a 3" is fired, the extra 1/4 of the casing can open into the throat of the barrel and cause a restriction. The pressure generated by a restricted 3" round can be higher than the older shotguns were made to handle.