I just picked up a Mossy 500 (with pistol grip) and I wanted to swap it out with the ProMag 6-Position Collapsible Buttstock Set. This set came in the mail the other day....with no installation instructions! Okay, so I can kind of figure out where everything is supposed to go but i need some help to identify these 2 parts (circled in red and bigger view).... What are they? and where do they go?/use for? ...Also, if anyone can please give some insight/tips on how to install this set correctly, that would be appreciated....i just don't want to miss anything.