Will the only hammers that have the serial number stamped on them were the Syracuse, L.C. Smith's and possibly some of the Transition ones from 1889-1900 in the 25000-29000 serial numbers.
The hammers on most are a snug fit and even with the screw removed are hard to come off. If there is that much play in the hammer to the tumbler it is a good chance that it was a replacement hammer or tumbler.