OK, that is the tool they use for farm machinery that use a sickle with removable teeth, like on combine pickup heads and grain swathers.
Used for changing the sickle sections in the machine without removing the full sickle out of the machine.
One side pushes out the mushroomed rivet, then you put in the new sickle section and install 2 new rivets and mushroom them over with the other side of the tool.
Makes for a quick turn around fix when a rock breaks a section out of the sickle. Back up and running quick again . . .