MDX 16.5. I would get an MD4 for the M91B, that way you can use 2x 18650 and get very solid runtimes at full output. I'm using a 9P w/ CR123 extender to run 2x18650, just like an MD4.
FWIW, Gene & co do lay out quite well what parts are compatible with what on their site.
In my opinion, the strength of the M91 platform is the spill. There is not a world of difference between the two aperture sizes, but the 16.5 does constrict your view a bit. I think a special drop in like an M91BN is worth putting in the 22.3.
Sounds like a good bet! Problem is, for the life of me I can’t find it on the Malkofff site. Probably just me but I emailed them to ask where it might be found or how I can get one. Also, anyone know...is the 16.5 lens and head gasket also compatible with 22.3?
Place an order for the 16.5 MDX, then just email Cathy. She'll make the substitution. As for the rest, I don't remember the 16.5 and 22.3 requiring different sized lenses or gaskets, but I don't have any on hand to compare. I'll defer to Thermal Guy on this point, since he's got more of a collection than I have at the moment.
I bought a 22mm expecting a big difference, but didn't see one. The 16.5mm head provides better protection for the lens, so if it's a working light, go with that.
You won’t see a huge difference but trust me do get better spill with the 22mm. And believe it or not in some cases more throw. I went to the 16.5 for my M61T Because I wanted to have the Crenellation. With the smaller opening I lost throw so I went back to the 22.3. Now that being said I still like the 16.5 better for some of my lights because only of the crenelations.