Fascinating! The only other O/U flintlock I've ever seen is at the Philadelphia Museum of Art......yes, where Rocky ran the steps. It's a Pennsylvania type rifle of approx. .36 caliber. Where it differs from the gun above is that it is made for the barrels to be rotated to be fired. Each barrel has it's own lock system attached and when that particular barrel is rotated beneath the gun, it is inverted. Really cool!