@nyx quite honestly, I am at a loss here. I think my brain is disconnecting slowly
If you don't have grub screws on the GEM28, I wonder why this is also happening on those mounts and if indeed tightening those 4 allen screws are sufficient or not. 4 allen screws should be sufficient to hold the two pieces together (not ideal, let's not go through that again ). Unless they did something equivalent the 4 allen screws of the CEM26... which I don't even want to imagine... On principle the less screws the better, so I would tend to favor the GEM28, but now I just don't know anymore.
I see what you are trying to do with the rings, and that sounds help fixing the issue.. but why is there any side-to-side play in the first place.. 4 holes, 4 screws, that should be rock solid. I am missing something here. I nearly want to dismantle the whole thing to understand what they did.
Judging from the video from the facebook group, the holes drilled on that nosepiece (GEM28) are not snug enough. Tightening the allen screws merely presses the nosepiece against the RA shaft.
What I don't get is, why doesn't the GEM28 have any grub screws?! The only reason I can imagine is that due to the extra 4 holes on the GEM28 "nosepiece", there wasn't any space left.