Had installed one of these today on a g&p m4 since the stock ARL is gouged when I opened it.
The modify ARL looks good as it has that E-clip that locks the ARL spring preventing it from missing once you disassemble the mechbox in half.
This E-clip would make this modify ARL out-of-spec from a regular one. Once you close the mechbox, this ARL is so tight that the spring wouldn't matter. I just removed this E-clip and it was fine. A good test was that flat screw slot on top, it's now moving freely with its spring tension.
This issue I had could only be isolated on my mechbox and not with other brands. But just an FYI if ever those of you will have issues after installing this with the E-clip...