I recently saw some pictures of a 4" Model 19-4 another gun owner was thinking about buying. The unusual feature about it was that it had a metal buttplate with a Commonwealth of Massachusetts crest installed.
I am sure that I have seen a discussion of this feature, probably on this forum (where else?) but cannot seem to find it.
Can some of the more well-versed S&W historians recall anything about this?
it's not uncommon. I have a 10-5 with a plate on it here to hand. Guys did it to keep from chipping their stocks getting in and out of cars, or otherwise banging their stocks into stuff. I've seen them with a variety of seals and engraving. Most are just brass or aluminum. They were not, repeat not, for doinking miscreants on the noggin.
These were very common when I was a police officer in Alabama in the 1970s and 1980s. Most were 1/4" thick brass, with some aluminum and stainless steel as well. Most had a S&W emblem engraved on them, though I did know a rather thin officer who had one on a M29. His was engraved "Barney".