Looks complete, likely for the 45MX series assuming the opening is 4x5.
Omega made one as well. I have one NIB labeled Mask Set. Picture below. I think the main difference will be in the centering/locating studs on the bottom. The omega negative stage has a slightly smaller opening, the four silver studs are used on the omega. Looks like there are four similar items on the Beseler just rotated.