What does "MB" stamp mean?

I cannot find what this stamp means on a US&S 1911A1 represents. It is a "MB" on a Marine Corps Barstow rebuild. Anyone know?

https://www.milsurps.com/images/impo...pg702455-1.jpg

You may find the information in the following recent link interesting.

https://www.1911forum.com/threads/5t...#post-13845559

Not saying the long held mystery is solved. But Magnaflux testing and subsequent inspection stamp does sound feasible.