My 59 WIAD is flat across with retaining ledges on each side, no raised center ridge like that, I don't remember if it also had a slight crown or not. I can look tonight. UPDATE: Memory is a fickle thing, and I guess I don't really look in there that often. I looked at mine and it is a smooth crowned wheel , no retaining edges . There is a small tension wheel directly above it that has the retaining edges. Probably what I was remembering.