I didn't know either until about 6 months ago when I bought a complete '75 Scamp parts car with a 4 speed. When I crawled under the car and saw it was cast iron I thought, cool a standard 833. But after I pulled the trans and saw the big retainer I knew OD, then I though DUH, the 3/4 lever was turned over. I should have noticed that right off the bat, but when I saw cast iron it just drew my attention away from the other things.
The site is great for learning new stuff, I am no spring chicken and still learn stuff all the time.