There are two of these, one more simple.
Are made of brass, I would suspect a calibration tool of sorts.
At the thumb screw on the side is a spring underneath for a pull-out quick release and insert a square device about 1/4 in size (pic two) a file end? (second pic)
I would say these were made to be used over and over and look well worn
The pencil-type hole on top is interesting, well it is all interesting.
In the first pic, there is a recess about the size of the quarter, this area looks like it was made to slide in under something for the setup.
