I bought this case in a local antique shop.
It is a 20x110 Hispano case made from steel, with very shiny chrome plating. There is no headstamp.
It has no primer and has never been fired. It was also likely never loaded. The primer pocket is boxer, which suggests US origin. The extractor groove is also deeper and a lot more rounded than any 20mm Hispano I have seen.
Has anyone ever seen one of these before? I would be interested in any leads on where it might have come from. I did think it may be part of an unfinished dummy round. The plating seems too good to be trench art, as the inside of the case and the primer pocket are evenly plated. This suggests that they were well prepared before plating, and looks as if the cases were purposely made this way.
If anyone wants one of these, send me a PM. The shop has eight more of them.