That postmark was on the same day of the 250th anniversary of Ben Franklin stamp (Scott #1073) that was issued in Philadelphia. Chances are Boston wanted to get into the act by having their own philatelic commemorative, and since the stamp was not issued there, the rubber stamp cachet was probably the next best thing.
For any of you interested in a 250th anniversary (1956) spread on Ben Franklin's history, Life magazine did a rather nice job at this link:
http://books.google.com/books?id=ez...ge&q&f=false