It's an MP5 style K-Grip! The original author provided two additional files - the short and the long locking tab. These press into place, and should probably be glued. I've printed a few of these now using carbon fiber filament, and they work good and function great!

This specific version has been modified slightly on the geometry to reduce the sharp edges that carbon fiber filament leaves, and has also been modified to accept flat head screws. I did not modify the locking tabs, so you may need to scale those from imperial if prompted by your slicer. I will fix this next pass I take at this file.

I tested durability on a knock off M-LOK rail I had kicking around in my parts bin, and I actually bent the rail before breaking the grip, so this guy is good to go with the proper print settings! I would avoid PLA if possible, but unless you're doing mag dumps you're likely to be fine. Please make sure you are following all applicable laws with vertical foregrips and SBRs/Pistols, yadda yadda.