The MB-4A Mount Block allows you to insert the hinge into the mount block and secure it with mounting screws. The cut-out is centered in the mount block.
The MB-4A-NS is very similar to the MB-4A, but differs because the cut-out is not centered but is offset to one side. The “near-side” wall is about 0.015″ inches thick.
The MB-4A-TS(HC) is also similar to the MB-4A, but now one wall of the mount block is basically removed and the hinge itself becomes the side of the mount block. This mount block also uses heli-coils to secure the mounting screws.
The MB-6AH, MB-6AL, and MB-6ATS(HC) are mount blocks which accommodate larger Soss-type hinges made by other manufacturers.