The NMLRA rules for smoothbore was made when Curly Gostomski and some of the others got together back in the 70's and decided to start building unrifled guns for use. They looked at the guns they could find and decided that since none of the ones that they saw had rear sights then that was the rule that they would use.
In the intervening 20 or 30 years we have come to understand that there were lots of smoothbores with rear sights, but the smoothbore rules haven't changed. Now we have the newer classification of smoothrifle. Which is a gun that looks like a "kentucky" rifle but has a rear sight.
I have seen rear sights on smoothbored military guns as well.
I guess the answers are:
1) Most original fowlers didn't have rear sights, but there were probably some with them since the guns were all hand made.
2) Many original "Kentucky" rifles weren't rifled, they were smoothbores.
3) If you shoot in competition, most clubs will follow the NMLRA rules and not allow a smoothbore to have a rear sight above the plane of the barrel.
4) If you have a rear sight on your smoothbore you can still compete in the rifle matches.
I was in a match yesterday shooting against three others. They were shooting rifles, one was a caplock and the other two were flinters. I was shooting a .60 smoothbore with no rear sight above the plane of the barrel. The targets were all clangers at 25 yards, the smallest clanger was the size of the bottom of a beer can. I hit all 7 gongs, and two of the rifles missed one a piece. Rear sights are for sissies. :blah: :rotf:
Many Klatch