I have never heard of a Dorchester Police Department. Boston Police has a district C-11 in Dorchester which is part of Boston. I don't know why a gun in 1938 would letter to the Dorchester Police Dept. At the very least I would think that maybe it meant instead of the armory it was sent right to the BPD Dorchester district. I will ask the BPD historian about that.
The initials on the butt looks like someone's initials and the date could be anything, a date they bought it or retired or who knows.

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