All,
There are a number of Vickers MG Manuals related to Tank and AFV mountings at this site:
https://vickersmg.blog/manual/tank-training/
The various .303 and .50 Vickers MG marks for use in Tanks and AFVs had cartridge ejection tubes fitted so that the ejection took place underneath the gun, forward and outside of the vehicle.
I am quite confident that the tray in question was the "cartridge case tray" mentioned in some of the linked documents.
One question that I have not worked through is why there appear to be 3 locations for belt boxes inside the turret. In:
https://vickersmg.files.wordpress.co...i-04-36-a1.pdf
under the description of "Mountings, M.G., No. 10, Mark 1" it states that
the mount was for "one .303-in. M.G. Mk. VI or IV and one .5-in. M.G. Mk. V. Alternately two .303-in. M.G. Mk VI or IV can be used in this mounting in light tanks, Mk V or VI." It goes on to state that "projections on the underside of the platforms are drilled for the belt box carrier"
Not having a good sense of the layout of the guns and trays for belt boxes, and also not knowing the dimensions of the trays, I wonder if the two outside trays are possibly used when two .303s were fitted and the central tray was used when a .50 was fitted?
Colin