I was just reading that Ford did use up to 700 Robertson (square drive) screws on each Model T bodies (at least on the Canadian models with bodies built by Fisher). (click for link)
Phillips screws didn't come into existence until the Model T ceased production, so it can't be Phillips.
Allen or hex screws were invented in 1892. I don't recall ever seeing Allen screws on a "T" but who knows...
That's my new guess...Allen screw.