From 1970 to 1995 the 200mm diameter drum brake systems used on BMW motorcycles were changed a few times. Shoes were narrowed in '81. A tube through the FD was introduced in 1983 to keep gear oil from leaking past the brake shaft (the shaft and O-rings changed), and in 1990 all models switched to symmetric shoes.
As far as I can tell, there are only three rear shoe sets: smooth-case twinshock (sold individually by BMW), finned-case and Monolever too 1990, and 1990-on. Due to parts supersessions there are many part numbers for these three types but they can be identified visually.
The details of the front drums, other than the shoes themselves, are not covered here.
"Shaft" and "cam" used interchangeably.
All Twinshock models '70 through '80 used the same brake shoes. The brake shaft bores in the final drive case and cover were sealed to each other by the cover gasket and the cams had no O-rings.`
With the finned case, the cover gasket did not seal the brake tube, so O-rings were used on the shaft to seal against the case and against the cover, which had bushings in them.
During 1983 BMW added a thin tube that was driven through the case and cover, sealing it completely. The shafts were changed to use thinner O-rings, which served to cushion the shaft within the tube and retain the grease. The cover changed, too. Also the K75 models were introduced with a drum brake.
Note: The replacement tubes supplied by BMW these days are likely to have a VERY rough internal finish which will destroy the thin O-rings.
This information is incomplete. For instance, the fiche does not show a different shaft for the Twinshock tube vs. non-tube design, whereas the Monolever models changed the shaft design to one with the fatter O-rings.
In 1990 all models switched to symmetric brake shoes, which also changed the cam. The Monolever shoes got wider. At this point, there were no more K-bikes with drum brakes. Below you can see how the asymmetric shoes are different and each bear on one wing of the cam, whereas the symmetric ones are the same. One shoe part instead of two, cam works in both directions, shoes have metal wear plates.
Although all models use the same shoe, BMW has issued at least three part numbers for it depending on which model you are looking under.
These are 15.8mm OD and 14.1~14.2mm ID. Actual length is 0.5mm shorter than nominal.