I've got a 631 which was originally was a 4 speed, no pto. It's got one now. It was originally a state tractor (made in 62) with a belly mower. I figure they bought a cheaper tractor to get it through budget, and them expensed the p.t.o. out to run the belly mower. Creative accounting.
I told a long time counter guy at an old Ford dealership what I had and he looked at me puzzled and said, I've never seen a 631!!