On AEG M4’s the bolt release is a purely aesthetic part – they don’t do anything functional. They are included on the guns because it completes the look, and without them the overall impression seems a little off.
Unfortunately this is a part that often goes missing when carrying work out on a gun, partly because of its size but also due to its unimportance. Even so, people tend to want to have it there. This part is manufactured by SHS out of steel, lending a little more authenticity to your replica.
This bolt release is only for M4/M16 AEG’s that have a pinned bolt. It can be modified to fit for the likes of G&P, but you need a lot of care & attention whenever modifying parts to fit.