Why are you making the adapter out of aluminum? George Blake's and RPM's are steel? I think I remember in the creation the adapter needed to be stronger than aluminum.
I can make them out of anything you want however there is no reason to over engineer it if not needed. I thought of using steel but the sample part is heavy in comparison of Aluminum which is light and strong.
I suspect that the treads are the weakest link in this assembly. Come to think of it some of the threads on the 10 mm bolt holes were stripped/cross thread. I examined them and maybe incorrectly assumed (hate to admit it but I did) that they had been cross threaded rather than stripped because I was not able to turn a 10 mm into the hole.
My thought is that this piece acts as a spacer as far as I can tell that doesn't receive significant compressional, tensional or torsional stress in regard to the aluminums strength.
Please feel free to share any concerns with me.