I can't help you there.Maybe the big knobs were not trying to make it confusing.I mean internally they are the same and the basic body's are the same just different inlet's.There were many different size's of inlet's I think without going and checking I have about 3 to 4 different size inlet pipes on the 3F.Maybe I guess this particle carby was only used on the Model 1 with the brass nut.The brass nut was a complete flop.The carby would never stay tight enough.Many people changed to the bigger inlet size.They easier to tighten and stay tight I guess this is why it is so hard to find the brass nut's today.

Here for a good time,not a long time.