If memory serves me correct, I believe it was called the "interchangeable" because parts weren't hand fit and supposedly parts could interchange from one gun to another, just like production made guns today. If you plan on shooting this, have the barrels checked out first to be sure they are safe. It may also have short chambers. Even if safe, I assume you know that you cannot fire modern off the shelf ammo through this unless it is something like Kent Game Bore or maybe Winchester low noise, low recoil shells.