I've done a lot of mental work on this subject and I think I know what really happened. The common wisdom is that it was caused by the shearing of the top of the guide rib. IMO that is only part of the answer. The Model Shop guns have sheared ribs and did not experience the 7th round stoppage, the problem only showed up after mass production was well underway. And only a very small percentage of rifles malfunctioned that way. Since they all had sheared ribs, most of them should have seen it. They didn't, the vast majority functioned perfectly.
Add into the mix that Art Tuttle always said it was "a stack-up of tolerances in the follower." When mass production started, machining was less careful than it was with the MS rifles made almost by hand under the personal supervision of JCG, a stickler for "machining to drawing." Workers were paid on a piecework basis, so there was a lot of incentive to turn out volume. In addition, there was an allowable variation from exact drawing dimensions that was acceptable. If you look at the blueprints, you will usually see a +/- figure after many dimensions that specified how far out was OK.
The upshot was that a rifle with a tight follower could tolerate the sheared rib and still function correctly. When a sloppy follower encountered a sheared rib, it experienced the stoppage. Once the rib top was built back up, a loose follower didn't cause a jam. I'm personally satisfied that is the answer.
