What kind of nut is used on Shimano hub dynamo with rollerbrake? I tried using the normal cap nut that is also used for my Shimano internal gear hub on my back wheel. The issue I have is that the axle on power side is quite short and the nut does not get enough "grip". When tightened, it jumps back. The axle on the brake side is much longer and the nut sits tight. 

Another thing that bothers me, is that the axle has a groove that looks like an anti rotation washer similar to the ones that come with the Nexus internal gear hubs belong there. I have spend a few hours looking on the Internet, but I cannot find any information on what kind of nut should go there.

The dynamo hubs model number is DH-C6000-3R.
I ordered a M9 cap nut and its fits.