Thank you Beachcomber.

I collect Boston Police guns, currently I have over 25 of them, all lettered to the Boston Police Dept, over 12 of them Smiths, all the way back to 1872.
The BPD refers the department. Over the years and models it has changed, I have seen BPD B.P.D. and on some Colts B-P-D
Boston is not the only Dept to use that marking, Binghamton NY for one used it as well as a Dept in Texas, probably others as well.
The BPD was followed by a rack number for identification, but had no relation to the serial number. For example a couple of my Model 64's serial numbers start with a number and then a letter and yet on the side of the frame it has the BPD and the rack number.
On my Model 36 the BPD is on the backstrap while on a Colt OP it is on the butt, which could be covered by the stocks.
Police gun collecting is fascinating whether you collect a single department or as I almost did a single model, such as the Model 10 or pre-10's or the (dare I say it here) the Colt Police Positive series of multiple departments.