Hi Bob,
I have part of the cfto for mounting the gpmg mount to the jeep tub.I also have the diagram that shows the mounting of the ammo tray.The first design had just a side plate where a strap goes through and holds the ammo box to the side.Later they modified that with bending and fabricating a proper tray that the ammo can drops in.I used to have both on my M38A1 CDN 2.I don't know if you can blow the picture up to see it more clearly?
Hope this may help you.
Derk.