This is a receiver jack used to evenly pull a freshly bedded barreled action from the stock.
NO MORE ROCKING!
Real simple. Install a scope rail (probably a milspec 1913/Picatinny type), install the studs and adapters to the rail, and slide the chassis over the action. The two studs poke through the slots and you install your brass washers and nuts. These work on an action up to 1.470" in width.
(I'm finishing up the chassis parts right now, don't have the hardware kits made for them yet)
Then just apply an even tension till the receiver lets go.
I made one of these years ago and gave it to Nesika when I left.