Mark,
Obviously, Walter and James Tyson are the experts on this. Until Walter responds, I will tell you what I think I know. I have owned numerous Ithaca Flues 28 gauges including two grade 2 ejector single trigger ones. About 12 years ago I researched the number of Ithaca 28 gauge guns. This included reading Walter's book and talking to him and James Tyson. My notes are not were I thought they were so the following is from memory to be corrected by Walter.
I don't think there is a record of the number of Flues 28 gauges produced. The number of NID 28 gauges has been published by Walter and I think the number of grade 2 NID 28 gauges is 58. I believe that Walter told me that he thought the number of grade 2 Flues 28 gauges was about the same or less. Its of interest to note that there were only about 20 in total of grade 3, 4, 5 and 7 NID 28 gauges produced.
Take Care,
Riprap
