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I have the disc final drive from my '79 R 100S (smooth case old style) sitting on the bench right next to a spare drum brake R 90 final drive. The disc F/D has the tapped hole for the brake shoe pivot and the hole for the actuating arm in the casting. The pivot hole has a threaded plug and the arm hole has a plastic cap on the outside. Here is the only difference in the disc brake F/D housing and the drum brake F/D housing that I see. The drum brake one has an additional ridge(no other way to describe it) on the inside face(brake drum and shoe side) that the brake shoes appear to rest on when brakes aren't applied and keeps them aligned properly. The ridge is not on the disc brake F/D housing that I took off my 1979 R 100S. I haven't tried to install drum brake shoes on it but I have to wonder if the lack of that ridge would make it impossible or unsafe to do so. I installed a later(ribbed/finned) drum brake F/D on the bike with same gearing to convert to drum brake.
I don't like the looks of the Snowflakes and I hear all these horror stories about them having flaws.