I bought both the CO-37 and the CO-137 when I was looking for a sheath to EDC my Arc4. While they recommend the CO-37, it will fit in a CO-137 albeit snugly at first. I just checked to see if the Micra will fit, and it fits my already stretched holster just fine. The CO-137 is Cordura while the 37 is plain nylon and a bit larger. Personal preference, I just like the Cordura better. Now the nylon one houses my Minox monocular. Buy both I'd say -- they are cheap enough and the one you don't use I'm sure you will find something to carry in it.
BTW that's with the twisty tailcap and no clip, couldn't track down my clickie ATM.