This is a (modified? / prototype?) breech for an Alien Revelation Electronic Paintball Marker.
This modified assembly uses a sliding trap door assembly. Parts are raw so this was likely modified from stock.
The Alien Revelation prototype premiered in 2003 and underwent testing and redesigns and ended up never being available to the public.
This example uses a sliding trap door, similar to an ice epic but adapted to to rotate movement 90 degrees. In the video we are looking at the top trap door, as a ball would ball into the breech.
On the left is where the barrel would threads it. One of the neat factors on the Alien Revelation is that the barrel sits incredibly far back. On the right (cut down unfortunately) is where a powerfeed style plug that can be rotated to block the air coming in on firing. When the black delrin (plastic) plate is removed you can see where the arm comes up from the ram to hook in and move the black plate to in turn, move the white trap door.
Andrew Gies commented on facebook that this Breech “looks modified or like the original design. My buddy Lane Baird has a working one. The ones I’ve seen from Griffan Spanner have a different trap door
See the embedded video below by Lane Baird below that shows how the different trap door that Andrew mentions functioned.
From the video it looks like this production (low production) breech / feedblock with trapdoor pivots from the tip (or chin of the alien head), rather than sliding similar to the ice epic design.
Thanks to Andrew for the input.