middle gear differences?

[skids]

Today I started out to replace an O-ring seal that resides near ther rear of the middle drive gear housing. I tried to use engine compression with an air impact wrench to remove the lock nut. Bad idea I think, because I that something "gave" and I guess the splines couldn't take it. Even in gear the yoke would not turn. I eventually gave up trying to get the damn nut off and decided to rob the middle gear unit from a spare 81H engine. I noticed that round the side cover has 4 bolts in it instead of 3(!) Can anyone tell me why? It is now installed to replace the broken(?) one.

[skids]

Trying to fix a small slow leak can sure turn into a mess. After thinking that I broke the 80's middle gear box, I decided to install the 81's in it. I ALSO messed up the middle gear drive from the 81H. The locating dowl was a little sloppy in the hole and while I was installing the unit, it became cock-eyed. I broke the upper right corner of the housing while installing it. OK, I figured that I would try to swap-out the guts of the 81 into the housing of the 80. Using a pipe wrench and a breaker bar, the lock nut was successfully removed from the 80's. Then, my mechanic friend showed-up and said that the pinion and spider gears are really difficult to line-up. Funny thing, the 80's unit seemed to be working just fine while lying on the floor. Maybe when I removed the three bolts from the rear of it, the gears were not properly meshing and that was the cause of the slipping. I peeked into the hole from the open right side of the unit and turned the yoke and all of the gears looked OK. I decided to just replace the o-ring and reinstall the 80's unit. I took it for a test ride and all is well. I HOPE I fixed the leak that I started out fixing in the first place!!! I would still like to know why the 81 has an extra bolt in the round left side plate.