Thanks Martin,

You are welcome Sir !!

I have seen your links, very helpful.

I try to save all the links that are good, and share them.

I saw now that the body of the old damaged transmission that I removed was painted gray. Someone painted all over including the attachments (kick down, all caps, modulator etc... ) That might indicate that it is a rebuilt unit?

Yes, very much so. Very sloppy to do that as my first rebuild I power washed the case, and it came out super clean, so no need to paint

Donor transmission to be prepared: Are you saying I better should seal it from the outside wherever I have easy access?

I'm saying the seals(o-rings) on the perimeter are the ones that are brittle hard/ cooked hard If you are not going to rebuild it change those outer o-rings.
Is there any other item to look at (B2?) before I Install this unit?
Maybe I can skip this but I don't want to regret it later. The W124 transmission access is very limited when installed.

I thought you had a replacement, and it is out of the vehicle? These early 722.3(some early ones) have a metal guide for the B2 piston that must be replaced to a plastic(nylon). To replace the the piston seal... with noting which way the dog-bone came out as it has a direction! One ball end has one stripe, and the other has two stripes. The ball ends are very slightly different. These dog-bones are sold in fractions of a millimeter for shifting, and getting it wrongly inserted may not up-shift. Here is link of a good DIY:

How to Replace Your Oil Cooler Lines

Frictions disks: I was buying the Precision banner kit because I was curious what is in it. It was $150 with various brands of frictions (I have more data here) . Maybe I bought this kit too early without knowing much about the rebuilding of the transmission. At least I have a set of seal in hand for sealing the donor trans.
For the real rebuilding of the damaged transmission I'd probably use Borg Warner frictions.

What I have noticed on American frictions that they are thinner. The thickness of what 100k mile Mercedes transmission friction would look like. The Aulomatic(which are in all these rebuild kits)brand is what is most of these kits. The Borg Warner's are great but thin too.. The oil groves in the friction are pressed-down, and I wonder with time if the oil does not cause them to swell closed(or expand a bit to limit oil cooling). With the Borg Warner's you will need new spring clips as they are too thin... I have learned my lesson, and only use Mercedes frictions as they are thicker, and the oil groves are machine-cut. You will probably not need to get new spring clips for the right gap for the clutch pack as the size will be like when this tranny left the factory...

You can just buy a seal kit which just has paper gaskets, and all the seals. Just buy Mercedes frictions. I have been buying mine from R*O*C*K*A*U*T*O those were made by TransTech.

Martin