This is a tool mount for the Accusize P900 series dial gauge. I bought mine from amazon.ca.
The bracket on the back of the gauge fits tightly in the slot, and a screw and nut hold it in place.
This can be used to level the machine by moving it in the X and Y axes and checking how far the needle moves.
Rotate so the large flat side is face down.
Do not use supports, or they will be a huge pain to get out of the slot. Even if there is a small bit of drooping while it bridges the top, it should fit fine.