Ok, so this is probably not a Masonic secret symbol. But I've had no luck figuring it out, and it seems like the sort of thing they would make.
I noticed this rosy marble slab back in October, while walking home from Quizzo. It's the doorstep of a place called "Grace Hall", but not the crew (rowing) Grace Hall on the Schuylkill River.
Since it's cryptic and made of stone, let's just say it's the Masons and leave it at that.
Found 10/06, in the area of Locust and 16th, Phila. PA
[Click photo for larger view]