If it's a longitude, then what is the J for? I work for a civil/survey company and have never seen anything like that. Of course, that was a long time ago. I wonder if it marked the location of an underground utility or was the mark for some kind of portable display or concession stand.
If it was a survey marker, there would be some kind of exact, identifiable point upon which to set the instrument. A marked, brass disk in the ground perhaps. You can see those all over Disney World.
It IS possible that there is a nail set in the pavement and that's it's identification number. We still use magnetic survey nails to mark corners, points, etc. But if that's true, that picture must be right around the time the survey was done because nobody would expect the paint to last in a heavy traffic area.