I think that we can forget this one as curious as it is. I am posting a picture but, upon close examination, the numbers are a different font and struck differently.
This cartridge and some others came the estate of an old gentleman who passed away a couple of years ago. He was a master machinist, an inventor and tinkerer with four patents to his name. I know that he made other cartridges and the guns to fire them. This may have been one of his other efforts.The bullet is lead.
I wish that I knew if he'd made a gun for this but those things were sold off shortly after his death in 2009.
Pic (not a great pic either):
Pete