I think I have the 50mm one that came from Surplus Shed (it was wonderful gift from CPF member and I did not ask what exactly it was), but the best focus comes when I have the head screwed out to where you just start seeing the o-ring, and yes, there is a small fit problem. I cannot screw the bezel ring all the way down. I believe some people are sanding down the head to make room for the extra depth of the aspheric versus the regular lense.
Mine rattles a bit. I think I put an extra o-ring in it and it settled down a little bit. In my opinion, we need to find a couple of aspherics that can be purchased today and 5 years from now and convince someone like fivemega to make a new bezel ring that will screw onto the standard Mag head normally and allow for perfect focal length from the LED to the back of the aspheric. I am just about ready to undertake this project but I have to play on my lathe first and see how good I can thread.....if not, I think I can make it look halfway good with some setscrews in the bezel.
So I don't think I aswered your question good enough so let me ask you to do a search on "aspheric sizes" or "aspheric fit" and see if that gives you the thread that I think jim jones has where he specifically talks about the 50 or 52mm aspheric from surplus shed. LuxLuther aslo has some very good threads on this specific subject.
Hope this helps.
Bob E.
