'72-'80 vs '71-back 904 converter differences

Bought a rebuilt torque converter for a '72 904 last month off EBay. Installed it in the trans today just seems a little off to me. If you push the converter all the way into the trans(farther than it would normally sit) it turns rough, like it's rubbing on something. The other odd thing about is is that I didn't have to grind one of the flex plate holes out that has to be done on Magnum flex plates.

Anyways, I noticed that most big name converter manufacturers list 904 converters for '72-'80 model years, so what changed from '71? The one I bought didn't specify a year, just that it was for a non-lockup trans.