Late response, but here it goes. The course did indeed open in 1948. Bell was able to secure some military projects in part because of his prior relationship with the Army Corps of Engineers as a turf consultant during the war. El Toro was another military project that sadly is now NLE.
As Patrick stated, there is much on the ground as well as aerials form the '50's that shows WPB's hand, such as the unusual serpentine like bunker around 1 green that is no longer there. In addition, there are a few bunker carcasses that are not anything like WFB ever did, such as the complex to the right of the 15th green complex. There are other things on the ground there as well, even today.
In addition, Francis did not join his fathers' business until after the war (still not sure on precisely when that was), but since the course was built in 1948, and the war ended in 1945, I'm not sure how much of an influence WFB would've had on MM, if at all. I feel very comfortable calling MM a WPB course.
Now, there has been work done to to the course since it was initially built, much of it for the worse, but that's another subject. Having said that, the course overall, it is quite solid and worth seeing, as there is not much original WPB left anywhere.
4,5,9, 12(from the right tee), 15 and 16 are very solid holes.