I misspoke in the last post. I did buy a 5.5 X.9 nipple from Trac of the Wolf to see if it would fit and it does not. Since the existing nipples measure .216" on the thread OD, and screw into the cylinder perfectly, I have to conclude there is no other nipple made, in a thread other than 12-24 or 5.5 X 1.05 (neither of which exists), that will fit. I talked with a local gunsmith who has been repairing black powder weapons for many years and he said if I brought the cylinder and nipples in he would look at it but from what I told him he thought a custom made nipple might be the only solution. I will do that when I get time someday but for now I wlll just leave the hammer in the empty recess. I have no intentions whatsoever to ever fire, or attempt to fire, the gun. It is, and will be, a safe queen.
Thanks to all of you for your ideas.