This is a top quality Osborne brand oblong / crew punch, manufactured in New Jersey, USA.
It is used to make a long slot shaped hole to fit a buckle onto a strap.
This one makes a hole that is 25mm long from end to end and 3.5mm 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 with a nylon board underneath to protect the cutting edge of your tool.
* Current photo is of the next size down, we'll replace this asap.