I have a 1917 S&W that accepts .45 auto rim rounds with aplomb. Full moon clips work also, but are a pain to load/unload without a tool. I found some original half moon clips and to my astonishment, they will NOT let the cylinder close. There is a small lug on the frame at the lower right rear of the cylinder opening, and the clips are hitting it. The half moons are straight and undamaged.
I've shot many a S&W with half moons in the past, but I've never seen this. Before I do something stupid, like grind/file, can anyone tell me the purpose of that little lug?