I have what I think is a Van Diemen 4x4 mattebox which I bought quite some time ago, in about 2002, from PEC in London. It's branded PEC, but I doubt they made it.
It was supplied with a ring to suit the filter thread on the front of what was then my main lens. The general arrangement is rather like a 83mm step-up ring with a chamfer on the outer edge, which engages with a tab and groove in the mattebox filter stage and is clamped in place with a thumbscrew with a conical tip.
I now need to get another ring for it. I can contact Van Diemen and ask, but it's a bit old.
Are these things in any sense standard? I'm not sure I've seen one quite like this and I fear it may be a Van Diemen-specific design that will be quite hard to find.
Thanks for any thoughts