The metric gas adjustment ring screws on and is held in position by that rectangular spring. Its a must. Also, the gas tube, once screwed into position is held in place by a small pin. Even small leaks in the gas system, coupled with numerous other factors like ammo, springs, etc., can affect functional reliability.
I couldn't get the original L1A1 gasblock to work (nothing lined up right) so I just put the South African one on there and haphazardly shoved some of the original parts on, the gas selector thing isn't held on by anything but the spring, so it comes loose regularly, and isn't very reliable, so I need the original parts.
I would have thought Century would have corrected that by now. I used one of their metric receivers for a project a few years ago and had to grind out that same clearance for the extractor.
There was one stupid problem I had though, they left a chunk of metal unmilled, which stopped the bolt from going all the way forward.