35-36 Ford pickups did not come with an inside mirror, only one outside mirror mounted on the driver's side. That's probably why the guy filled and sanded the holes where a PO had mounted one because he knew they did not come from the factory with an inside mirror.
Henry probably thought, hey, this is a work truck and the bed is gonna be full of "stuff" and the driver won't be able to see out of the back window anyhow, so we can save 5¢ per vehicle. That's probably why trucks didn't come with white walls as well. These were WORK trucks.
I believe 37 was the first year that came with an inside-mounted mirror, but I'll let the 37 pickup experts answer that one.
FWIW, I added an inside mirror on Apatchy for safety reasons. I am 5'8' and I mounted a 46-48 style mirror in the center of the header panel over the windshield where said panel curves under toward the windshield and it suits me just fine. Looks nice as well.