F for Fire
This valve cover in Tribeca has the enigmatic inscription "F. " This is presumably for "Fire" and is either part of the fire alarm system, or part of the extensive system of high pressure water pipes installed in lower Manhattan in the early 20th century to aid fire fighters.
A simple, single strand basket weave design, with squared strands.
Just as today there is usually a small square DWS or WSNY valve cover near every fire hydrant, this cover probably stood near one of the now-removed HPFS hydrants.
This cover was on Washington St. between W Houston and Spring. Another can be seen near Foley Square at Leonard Street and Benson Place.