This is a depth gauge stand for a Replicator 1, Replicator 2, or a Wanhao Duplicator 4 3D printer. I assume it can also be customized to work with any box type FFF printer where the extruder is mounted on 2 parallel rails.
The bridge sits on the rails, and a depth gauge fits through the hole in the middle. This enables one to get height readings quickly of the height of the build plate.
UPDATE: I tried it. It kinda works. It doesn't wrap around the rails nicely, and needs some pressure. The problem is, the pressure required bends the rails too. I ended up using a piece of wood with a hole drilled in it.
Print upside down, so that the top part is flat and overhangs don't happen.