The ammo tray on mine was purely there out of neccessity and was home made by me. My GPMG is converted to 8mmX63, which comes on cloth 250 round belts which would not fit in the 7.62 ammo can, so I made up the tray to accept the swedish cardboard box. As I mentioned previously, there were the odd trays around, but they were a local thing. The .308 ammo can locks into the existing arm quite nicely.
Here is a quick snapshot of the CFTO detailing the modification for the link and cartridge catching bags. The bags are found in various surplus stores (I last saw them at prefair in Montreal) and I think Brian Asbury may have had them listed at one time as well. I have a set or two, but have never got around to making these items. Quite frankly, they filled up too quickly anyway.
If there are any other details you want to see, don't hesitate to ask. I realize this one photo may leave a few questions.