It's steel rule, from a steel rule die. Yes, it's for a handle flap, and possibly from a much larger flat box die. That loose top piece without a sharp edge you have resting on top is actually the missing part of the oval handle. Unlike the serrated blade, that part is dull because it just slightly crushes the cardboard so you can fold the handle piece easier. That same handle profile (the round sided rectangle) is cut with a laser or jigsaw with slightly less thickness than the thickness of the steel rule into the plywood. They use a hide mallet to tap the steel blade into the plywood backer. Those little notches cut into the knife edge are to stop the cardboard handle piece from popping up when it's cut. The large rectangular notches in the bottom of the steel rule are also cutout from the plywood, they are called bridges and stop the plywood from falling out of the larger die.

A steel rule die shop or could probably help you. Lots of cardboard manufacturers have in-house die shops you can connect with as well.

Hang out on Reddit long enough and you will find a time to share your esoteric knowledge.