Is the lower barrel supposed to fit flush with the centerband or should it go into the centerband a little bit to hide where the two barrels meet?
The lower barrel should just fit into a little recess in the centerband.
Smitty had it right if you need to cut a tenon.
I think they had three different styles of center bushings. One had an adjustable piece on it and one had a small step on it. The latter bushing needs to be reversed during the turning process. Either one of these were used for cutting a tenon. The adjustable one was used so you didn't have to place the centerband on the bushing. You would use it in place of it.
The newest designer kit they carry doesn't require a tenon so the center bushing now just has one large step on it. What is the PSI number of the bushing kit(s) you have? What is the number of the pen kit?