Is this the type of carby you were asking about.The smaller end is where the brass nut was fitted to it and went to the inlet on the bore.It came of the Model 1 Rotomo from 1955-56.I am fairly sure it was called a 3F as the paper work that I have seen calls it a (Carby Victa 3F).It was just known as the one with the cir-clip.