early M1 Rifle receivers with some manufacturing differences

Starting in the late 30K serial number range into the early 40K serial number range there are some of these "extra wide rib" receivers, all are revision 1 and REP heat lot codes. To date
there are about 12 known to collectors and more than half have had the rib weld up for the low rib correction. The reason for these extra wide rib receivers is still not clear !

Later (last photo) there was a small narrow rib on some revision 2 receivers such as this in the 48K serial number range.

If you have or know about a receiver with this extra wide rib, it would be appreciated if you
could post the serial number or photo.

First photo shows the extra wide rib receiverAttachment 54215Attachment 54216Attachment 54217Attachment 54218

SA was scrambling to solve the 7th round stoppage right at this point. According to Art Tuttle, the major problem was a stack-up of tolerances in the follower that allowed it to move after the first 6 rounds were fired. Extra guide ribs and thicker ribs were tried as a means of holding it in place. When the contributing factor of the sheared rib top was identified, these ribs were not needed and were dropped.

Serial number 36733, Rev. 1, REP 5. 7th round stoppage modified.

https://www.milsurps.com/images/impo.../2dubfnn-1.jpg