It is common when engineering changes take place that there is a transitional period when S&W mixes and matches parts to use up supplies of older version parts... Most of the -1 changes took place in the slide, the frames of the 4506 were perfectly good, economically S&W's attitude was/is why ash can a usable frame just because the new engineering change calls for a round trigger guard?

There are even some really early transitional versions running around where 4506 model marked frames had a 4506-1 slide slapped on them.

__________________
Conrad
SWCA #1830 SWHF #222