Great info David; I do as you stated on the "J" springs; rotate them 90 degrees to lock the small retainer part of the spring in the slot of the brushplate; still not easy for me. Shimming the older armature for the newer brush plate is a great idea. As stated, the number of shims for the tolerance is a given. I do not do repairs on these often and I forget what I have done in the past.
On the e units, if all the fingers, drum, pawl are intact but the e unit is sticky, I shoot it with WD40, let it soak a bit, cycle it a bit, then blow it clean; works pretty good. Picked this up many years ago from a good AF tech. If it does not work then, I take it apart and check for other issues. Also works well on Lionel e units that look kinda dirty; sometimes it takes a couple of tries but surprising how well they work after a WD40 shower.
One thing on AF, I am always learning from folks like you; always. Thanks much for great info; Falcon70
