Kutter, your comments are very pertinent. However, the rib need not conform to an irregular receiver on some 61s. My early prewar gun has a receiver that is absolutely square to the barrel with no steps. I don't have a later gun to examine. If a rib were installed on my gun flush with the receiver, the barrel could still be removed. I think the curve of the receiver matching to a 1/4" rib is a non issue. There isn't much of a curve in a quarter inch. Thanks for your great post. Bill Murphy