The outer mounts were a piece of U-shaped metal. The sides of the U give it extra strength against bending. The washer that goes on top of the mount has trimmed sides so that it fits in the bottom of the U.
If you use the tail-shift outer rubber mounts on a side-shift bar, it will sag and interfere with the rear shift rod. You won't be able to get some gears. Not usually regarded as a good thing...
--DD