Richard,
Not sure what you mean by a step, are you talking about the one on the outside of some of the gutserts? That is the way I do it, but I have seen some that had no steps on either the inside or outside. Not sure where I got those from, but those where the ones that I like and used it as a model for for the ones I did.
When I was making my own I used my own O rings and kind of modified them to fit the guts that I had bought. If I remember correctly they were an odd size, off something like .006 from the standard. Now I pass on the goose calls, not my thing.
Marvin