Here is a scan of the 20x139 Headstamp I have so far had absolutely no luck identifying.
In case you have difficulty seeing it (it was an awkward round to scan, as it is painted black). The case headstamp says : “20mm D 00 VM” and the primer says “DM 64 343 D01”
The case looks pretty tatty in the scan. When I got it it looked like that. I presume the marks came from the gun, as I have four more of these cases and they all have the same marks on the head.
The general opinion is that this is possibly South African. If anyone has any more info, I would be grateful. Where do I go next to try and identify it if no-one on the forum can answer my questions?