David:
I suspect that the stamping "City of Philadelphia" is an id of the dealer who sold them, and does not mean these were owned by the City. The NEW YORK address on a colt doesn't mean NEW YORK owned them.
I believe this same fellow put similar marks on derringer copies he sold.
This is based on some past research not at hand, but follow up and see if this doesn't turn out to be marking put on various types of arms by this dealer. JEff