That floating Breech Lock Cam has been a mystery for many years. That mystery was compounded for me when Bill Ricca posted a clip from one U.S. manual that specified that guns built by Saginaw could have the Cam drawn tight before staking, while all other mfgs were to back off the screw slightly, then staked.
Then I learned why. For those interested, it's all in Dolf's book. It's easy to miss, and the significance is not so obvious unless you study the section describing the Ordnance Dept's efforts to overcome the apparent weaknesses of the original 1917 and the aircraft guns based on that design. Once they figured out why receivers were failing, one contractor developed the solution. That is, the floating cam. Thank Marlin-Rockwell for that little bit of magic. See Dolf's Vol 1 page 230, the section on Endurance Testing of Browning Aircraft guns.
As to Saginaw having an exemption, my guess is that the strength of the Armasteel bottom plates proved to be greater than the forged ones, and that allowed them to stand up better to the beating that the floating cam eliminated. I think the stirrup bottom plate is not prone to cracking much anyway, unlike the original, dovetailed design. But the floating cam stuck throughout the generations of development.