Decided to start a new thread on this subject.
I have a similar item picked out of a box many years ago - clearly this was an accidental loading. I also have one in .38" Enfield MkII. Folklore states that there were some deliberate SP loadings for special use made by inverting the jacket - but I have no documentary evidence. Mispacking did happen - my one and only 9mm MkII Daily Proof of Work (green primer annulus) came out of a box of regular ball ammo………
I have never heard of a “proof of work” cartridge. I have never documented a green pa on a British 9x19mm, though I have seen green pa on Canadian 9x19mms from WWII.
Mannlicher, could you tell us more about this round (headstamp, etc, and more about the use of “proof of word” rounds? A photo would be great!