I just looked at a cigar pen I have and came up with a question and teo,(Not one but Two possible solutions/answers.fixes)
The question is did you remember to put the gold "trim piece" on the big honker piece of powder coated brass that goes in the bottom of the upper tube?
If you didn't then that would explain a gap at that joint.
If you didbut still have a gap then that would be the point to add the opps ring.
If you did mill the upper section too short all is not lost(Ed I am surprised you haven't figured this out seeing how many cigar pens you turn[
])
Dismatle the top section.You will need a punch that will "angle in from the center band end and hit the "stud finial" out.
It is a littler tricky but can be done(DAMHIKT).
REMOVE ANY jewlery you have on the hand that you are using to hold the pen barrle.(another DAMHIKT).
Once you get that piece out, lay all the components out like they go together and determine how much of the brass tube on the end of the piece that the finial screws on to you will have to remove.The transmission slides up into it but you don't need all that is there.
If you burr the inside edges make sure to clean them up.
The big black piece only serves as a guide for the transmission.I doubt seriously that piece and the transmission finial are bottoming out but you could check that.