I happen to have one extra lifetime promo code for Gun Disassembly 2 which unlocks every gun, including future updates (Windows and Android only). I will give it to the first person who can identify what the function is of the parts in the above image and what gun they are from. Post your guesses in the comments below. One guess per person.

Update: JAG2955 was the first person to correctly name the part. It is a FN FAL bolt hold open. He got it just before Sándor and Jake (on our Facebook page).