Interesting. I’ve made cases like that by accident in a sizing die and I bet I could make one on purpose. But for our purposes here I’ll assume it wasn’t made deliberately, such as for a joke.
It almost looks like a case that collapsed when seating the bullet. This might also account for the steep shoulder angle. Perhaps it was originally something on the order of a 40 degree shoulder that was pushed back in trying to seat a tight bullet and the case collapsing too.
Just SWAGs on my part.
Why not gently pull the bullet to see what it looks like on the inside>