This is an economy oblong hole punch, known in the saddlery trade as a crew punch.
It is used to make a long slot shaped hole to fit a buckle onto a strap.
This one makes a hole that is 16mm long from end to end and 3mm across the width. You can also use this tool to make a longer slot, by moving it along slightly and hitting it again.
This punch is used with a mallet, and we recommend that you use a nylon board underneath, to keep the cutting edge of your tool sharp.