I'm looking for a front sling mount on my front gas block (M&P15 OR). I was surprised when I got home from the store that my M&P15 OR did not come with one stock (for the price I think that's a bit crazy). I'm still very ignorant to all things AR-15, but I see on the bottom of my gas block and there are two holes. One that is about 1/8"across and is threaded, and one that is ~5/16" across and has a recessed grove just inside the lip of the hole. I'm running a Blackhawd three-point sling right now and don't absolutely need the mount but still like one. Two questions, where can I get a threaded sling mount for the smaller hole and what is the second hole? Thanks!