Don't know and I only have the K6 600 service manual. It might be to seal a passage that sprays oil onto the stator to cool it.
P.S. The fiche says that #20 is an oil gallery plug.
P.P.S. I just noticed that the K9 1000 has such a plug. The Engine Lubrication System diagrams show that the oil gallery feeds a passage through the stator cover to the crankshaft(!). This is how the oil passages in the crankshaft and the main/rod bearings are lubricated. Possibly that's what you have. The distinguishing feature for this system is that the bolt that mounts the rotor to the crankshaft is hollow and its head has an extension that engages a seal on the stator cover.