This flap is clearly intended to support the projectile as it is moved into the hoist. At the lower level, the flat does this. At the upper level the only other support would be the fixed pawl which would not give enough support.
My question: What raises and lowers this flap?
Something must raise it before a projectile from the lower flat passes by it. The flap cannot be moved manually. It is too heavy and not shaped to be pushed aside by a projectile.
My suspicion is that raising the pawl carrier causes the flap to go up and maybe opening the aperture shutters causes it to go down but these are only guesses.
I can find no reference to this flap in OP-769. In fact, it's figure showing the upper flat aperture looks nothing like the real thing. The famous training video shows the lower flat.
Maybe someone here actually knows.