including the 350F for 5-minute oven test

you will know that it's M

Leather: Smooth

Nickle plated solid brass casual buckle

professional looks

Raven Concealment TopStop (AR Upper Receiver Cover) Truglo including the 350F for 5-minuteRaven Concealment TopStop (AR Upper Receiver Cover) The TopStop cover is designed to snap on most AR15 or M4 type upper receivers when they are detached from the lower receiver. The TopStop completely seals the upper receiver preventing fouling from getting in or out and it stops the bolt carrier group and charging handle from sliding out as the upper is moved around. The TopStop is made out of super tough polymer and is impervious to most solvents