The plastic ring in the bottom of my PM-6 sheath is a clever idea for keeping the pushbutton from being depressed. It looks like a PVC pipe, threaded endcap. The center of which has been drilled out (or it was manufactured this way) to clear the pushbutton on the end of the M6.
It also protects the soft rubber pushbutton cover from abrasion by the coarse, nylon cordura.
The kydex-like, thin plastic "tunnel" probably serves more than one function and is easy to remove or modify, according to one's preference. I left it in till I make up my mind.
The sheath IS a very snug fit, even if the flap comes unsnapped and you stand on your head, the light ain't goin' anywhere.
BC