This is a 2 part hammer tool, used for fitting our 5.5mm size eyelets. (Internal diameter)
To use this tool, push your eyelet through a hole in your leather or fabric and sit it face down into the recess in the base half of the tool. Put the point of the handle into the centre of the eyelet, and hit with a mallet to secure.
This is the larger of our two sizes. You can find a tool to fit 4mm rivets here