Here are pix of my home-made adaptation:
http://www.flickr.com/photos/4200284...th/5913717501/
It didn't seem to make a noticeable difference that I did not do everthing in brass.
Just save the original parts in case you ever do get a good 21AX.
Edit: note that the pieces near the face are the home-made part, the mounting ring and straps are from a later chassis - but you could fabricate something similar, I think. I also recall that the small hairpin magnets were helpful in getting uniform purity.