It looks like a nozzle plug which covers the combustion chamber. A closer look at the shot does not show any holes in the object so it cannot be the injector plate. In the photo you can't see the converging part of the engine so it can't cover the injector plate.
The Energia that is in the video was a structural boilerplate with real engines, according to Orbiter Forum:
Just read the link - it's not the real thing, but a dynamic test mock-up. Essentially a boilerplate rocket with real engines - they were in the middle of testing the design when the program folded.
They never fired these engines as it was only a structural mockup and wasn't designed to be flown, kind of like SA-500D which was only structural and never was used for firing engines. Hopefully this clarifies things.