Re: what is this?

[Don]

V4 engines made before 1973 had an interesting way of being attached to the boat. Two stern clamps/slides were the only thing bolted to the boat. The engine simply slid down into these two clamps on the transom and was locked in place with two threaded pins.
The clamp you have is a way to secure these older engines on stands/racks, etc. Simply clamp that bracket to the stand, then slide the engine down into the clamp. I surely wouldn’t toss that out, perhaps someone else might have a use for it.