In 1984, Jess Briley threaded a 21" vent rib Ithaca Mag 10 Roadblocker for me and made 3 tubes, IC, Modified, and Xfull Turkey. He told me at that time that they were the first tubes he had made for a 10 gauge Ithaca and I don't have a clue as to what the i.d. was on any of the tubes and they weren't marked w/ the constriction.