Bendix master vac power brake set up

It's a check valve....or at least it once was. Looks like the stock one to me. Good luck finding an NOS or even a good used one. Unless you need it for a restoration, I'd pull that thing off along with the booster and reservoir and make the car manual brakes. Those boosters were nothing but trouble and the repair parts are getting pretty hard to find the last time I looked. I'd convert to a dual hydraulic master cylinder while I was at it. Just get one with a small piston diameter if you want an easier pedal.