I'm no expert, but I think it is a "shot and ball" gun. To me, the rear sight looks exactly like a sight with both leaves made to fold down. If you look closely at the picture you can see the recess where the erect leaf goes when folded. With both leaves down it is ready to shoot "shot". With one or the other leaf up it is ready to shoot "ball". Perhaps "leaf sights for 100 and 150 yards" is a misprint. It would make more sense if that was supposed to read "leaf sights for 50 and 100 yards". It was not unusual for this type of gun to have three folding leaves: 50, 100, 150 yards. So, another possibility is that the owner was optimistic and, with only two leaves, wanted 100 and 150 yards.

~