The fibers from the clutch discs would mix with the oil and plug the hole without that cotter pin spinning around in there toi keep it open. I think those same engineers were still there in the 1980s because my dad bought a new 1982 GT Mustang and it had shield over the exhaust crossover pipe, so that when the rear seal started leaking oil at around 20,000 miles, it would drip onto the shield and then onto ground instead of dipping directly onto the exhaust and burning. In 1983 they went to a one-piece seal and finally fixed the rear seal problem.
The Ford 8N tractors have them too.