It looks like a miter vise......
With a custom hold down but not sure what the squared off pin is under the hold down.
Your pics make it hard to see what is going on. Stand back and take better pictures.
Put a piece of wood in and clamp it and then take a photo. It appears the lever on the bottom pushes up on the squared off pin?
It must be a custom fixture for some unknown purpose involving miter assembly. I assume you already knew that though.
Take off the extra stuff and you have a nice miter vise which is very handy.