I would like to the XM-L in the H501 though, I don't see why not since their is no reflector.
The reason why you can't do that is because the H501 series uses an optic to spread the beam evenly over a wide area. ZL faces some challenging situation when trying to change the XR-E within to an XP-G. They then dropped the idea and opted to use a reflector with diffusing lens on the H51 series. XM-L would pose a much bigger challenge when used in the H501 body.
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