What "exactly" is this used for?
There is not much at all on the internet for this tool. I found a picture of one with a machinist rule sticking out of the face, but I am confident this is not a square like that. The face does open for something to be inserted inside, but there are no measurement markings anywhere on the tool and the face does not rotate. There is a small slot at the bottom center where the "feet" are, but the slot has a 45 degree angle to it, so what ever is inserted will not stay flush at a 90 degree angle along the edge of the slot.
I have had one person say it is a specialty tool for fabricating breathing apparatus tanks, but they were speculating.
Any guesses, besides a paper weight?