It's possibly something that's been made specially for the prop.
The BBC's props people tend not to build props using just "found" items (there are quite a few in the "Classic" series, guns made from hosepipe parts, etc.), it always seems to be a mix of "found" items and specially made bits (3D printing, CNC milling, laser cutting, all that sort of business). I'm sure they do it deliberately to confound replica builders. They'll have to try harder ... !! :lol
Sean, I don't suppose you could send me a set of the images you made, could you? It would be most handy for reference!