The first announcement that you are approaching the town of Florence is the whitewashed "F" on a small pyramid-shaped hill of burned-looking rocks. James W. Clarke, Last Rampage: The Escape of Gary Tison (1988), pp. 14-15.
On top of Poston's Butte is the tomb of Poston himself. He had intended to build a temple to the sun. But a pyramid -- and an F -- will have to do.
(Update, 08sep2004 -- More Poston information here)
I think it says "01" to the right of the F. Not sure.