Here's a curious little item that I unearthed in Bryan's collection: a weapon platform for Rogue Trader era Space Orks:
The gunner and weapon are standard issue from the Ork War Buggy of the period. It's the platform that is unreleased. Here is a view from the top-down:
You can see the hole for the weapon mounting on the left, and two indents where the gunner's feet sit on the right.
Here is a view of the underside:
Are those three discs on the perimeter some sort of anti-grav devices that allow the platform to hover? Have you ever come across rules or descriptions for this in one of the old Rogue Trader rulebooks?
So it may not be much to look at, but this particular collector thinks it's a lovely little piece of kit.