@neutralwhite
The h/l ring switch is to take the brightness from high to low. I think low is an estimated ~10-20 lumens?
The head tight is high, the head a light-touch loosened is low.
If the switch fails then it's always high. It also secures the drop-in to the head...so a switch -- whether plain or h/l -- will be required.
The h/l is gold colored, the non h/l switch is silver.
I'll measure the MD2 glass diameter so you can see what size filter you need between the glass....your best bet might be to redneck engineer a filter that goes on the outside. It definitely needs to be larger than 22.3mm or it'll fall through the head's opening.
It appears that size and battery run-time will be the major differences in the MDC 16650 and MD2.
You should be able to take out the MD2 UV drop-in, where as the MDC16650 will not be removable.
I imagine the beam on the MD2 will be wider, since the head is .25" wider....but I don't know that for certain.
The 16650 does have the sweet pocket clip. But MD2 has a couple clip options...I prefer the import light clips.