Looks like someone refinished a fairly early Utica gun and remarked it as a Philadelphia gun. The highest serial number Philadelphia era 20-gauge Sterlingworth I've recorded is 262,867, but I have recorded Savage guns as low as 259,354 which was a 30-inch 20-gauge, a slow mover for sales back in the day.
You don't see that engraved Fox Proof on Sterlingworths!! That is only see on XE-Grades and higher. And, they left the Savage-Era Fox-Sterlingworth on the sides of the frame. But, other then that she's cherry!! Not!
I'd bet it barrels originally read like the bottom set here --
http://www.foxcollectors.com/barrel_stamps.htm
