Ronald M
Veteran
I don't think the question is "What's it for?" but rather "Why's it so big?" because I know that it's not so big on the M3/2/4.
Because the a small hole was all that was needed to access a simple almost jewelers screw.
I have seen the hole behind the big red dot and I submit it is large to accommodate the special big oversize tool now required to make the adjustment. To plug a big hole, one needs a big screw.