Yes big mystery for me too... I will buy a gen myself and try to figure this out.
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I've sold my franken UPO (with rep bezel) now but when looking at fitting a gen bezel the work I see you would have to do would be very extensive.
The only two ways I could figure were - not necessarily in the correct order.
1st method
machine a ring to space out the case to fit the dia of the bezel
turn a groove in the side of the ring to take the bezel retaining spring
That's the easy bit with the correct equipment. Now the hard bit even with the correct equipment, very easy to spoil the case.
Machine the case by turning a groove, along with the anti rotation hole to take a gen bezel spring from the XL. This has to be done as the serrations on the gen bezel are at the outer dia they will completely miss where the rep spring is which is now covered by the new spacer ring anyway.
I suppose you could fashion a new spring device, thus eliminating the need to machine a full groove, to allow the bezel to turn one way and not the other but the chances of getting the bezel to line up would be very hard.
2nd method
The only other way I could see would be to machine down a rep bezel to leave a section with the serrations and somehow fix this to the gen bezel.
I could not see any other methods, apart from just swapping the insert, but I would love to see how Uwe did it.
I would be very interested if any one else has any other thoughts on other methods.
Inushuk
I completely understand that you would not want to remove the bezel but if you ever have the need to, take some photos and solve the mystery!
Oh and yours looks fantastic.