A four-screw framed revolver that is stamped -2 would certainly be an oddity, but would also be incorrectly stamped. A Model 28 with a serial number in the S221000 range certainly could be a four-screw as this frame was made to about S228000. It may also be marked MOD-28, have a four-screw frame, but have an extractor rod with a left-hand thread, and would be configured like a -1 model variation. This serial number is right at the transition to the -1 and shortly thereafter, the -2 model variation.

Bill