It'd be the date stamping... Nikon did do some production changes on the F3 over the years. Early cameras had a basketweave appearance to the rubber grip that was slightly shiny. They also used a flatter AE Lock button on the early cameras. So, if your camera has either one of those two items, it's a March 1981 build. If the grip is smooth and the button is domed, then it's probably a March 1991 body. Serial would've been either 132xxxx-133xxxx for 3/81, or 192-193xxxx for 3/91.
Mine has a smooth grip and a bump on the lock button, so March 91 it is then.