I obtained a Smith 64-3 this spring that had "BPDxxxS" stamped on the left side of the frame. Obviously a police trade in, does anyone have an idea which PD (or departments) this gun may have come from?

Did you get it from a LGS? If so, maybe they can tell you the source/location of their police trade in buys. BPD is too common and a best guess by anyone. I purchased a Sig police trade and was able to obtain the state location only from calling SigSauer CS. It happened another time with a 3913 purchase at Gander Mountain. "Maybe" S&W CS could help. Good luck trying an internet search.

Boston Police Dept. Ours were marked just like that. Let me know if you want to sell or swap it.