Hmmm... I find this quite odd.
It turns out there are two versions of the MB, plus one version of the non-MB, for the 4x5 F2.
One MB type (which I do not have) has a large feature running the full length of the wide side of the frame,
on the outside, with a silverish lever associated. This sticks out about 1/2"? on the left when in portrait mode.
The other version (which I have) has a similar feature but only about an inch and a half long, at the
upper left of the frame in portrait mode. It has a similar lever that pulls out to the left.
They both look quite different from the non-MB (which I also have).
Download a copy of Sinar Code from Butkus or wherever. Look in the right-hand column of page 11.