you got my curiosity up. I checked the vent plate on my 1970 Caravel
and 1972 Overlander, both mount with 4 screws and none of them are long.
I do recall my 1977 Safari having six screws in the vent plate (long since removed). It seems there was some connection to the inner skin, which is usually 1-5/8" to 1-3/4" (which would require a 2" to 2-1/4" screw). If a long screw(s) is required, I would assume it's to ensure that the intake and vent for the heater remain securely in the holes in the inside skin, so that they are exposed only to outside air.