The slide on the bottom of the tray is definitely original, and I think Colin will find that when the turret is rotated 90 degrees to the left the back edge of it will line up nicely with the edge of the hull so that it can be emptied on to the ground or for the more conscientious crew, into a container placed on the flat top of the left side fender. I would think the hinged front panel was a later addition when they found the casings would sometimes bounce out the front.
Brilliant work Colin.
