Reading about the throttle slide blocks (gleitsteine fur den gasgriff ) I thought I would give you my experience with a similar part but for my R25, being a single pull cable. I purchased from Ideal Seitenwagen in Berlin, a complete throttle control unit with front brake and also the clutch control unit. They both looked very good. However, actually using them threw up two major problems. In England we say they look good stood still.
The gleitsteine seemed to be made of what we call monkey metal, a very poor alloy. The result was that after about 500 miles of use, of the sliding parts that fitted into the twist grip was so badly worn, even though it had grease all around it, the throttle twist grip would move perhaps 5 mm. before anything happened.
I contacting the people in Berlin, and for a €10.00 note,, they sent me a replacement. This was even worse than the original, there were holes in the casting, caused I would think by air bubbles, when the metal was injected.
So the next thing I did was contact Mark Huggett in Switzerland, explaining the problem. He told me his parts were made to BMW standard, using the correct metal, so I purchased a similar item from him, but with import duty /postage/ exchange rate, it came out to about £40.00, however it works perfectly, and now after 1500 miles it shows no sign of wear.
The clutch lever also had problems. Again after about 500 miles, the clutch cable SNAPPED ! very close, to the actual end piece (nipple) that pivots in the lever. I examined the lever with a new cable, and found that the lever had not been cut deep enough inside allowing the cable to always pull in a straight direction, the lever was used. This meant in pulling the clutch lever the cable actually bent, at the nipple, and that caused it to snap.
Using a suitable diameter drill bit, I was able to grind down, inside the lever to allow the cable to pull in a straight line at whatever angle. Another problem solved.
The conclusion is, these parts look good, if you are not riding the BMW, (stood still) but for practical use on the road they are not really up to the job.
If you are using similar parts to these, it might be worth having a look at both clutch and throttle controls, based on my observations above