The difference is the the gate latches. the 72-80 latches to a slot that is welded to thebed it kind of looks like the same way the door to your house latches. The gate for the 81-93 latches to a striker post like the one your door to your truck latches to.
To put a 70's gate on an 80's you would have to weld on the tabs for the gate to latch.