It's double stack. Must be for a carbine.
The only release lever notch is in the back, so I'm thinking it must go to something with a rear release similar to an AK47.
This is very puzzling!
2A: "A well regulated militia, being necessary to the security of a free state, the right of the people to keep and bear arms, shall not be infringed."
US Army Veteran: OIF and OND. NRA Lifetime Member.