Hmm, so, it's possible that there never was a # 1,2,6 and 8 adjustable nib? That is odd, as, from what you are saying, the adjustable nib range started on its own after the gold seal nibs. If one doesn't include the later un numbered adjustable nibs withe fully covered slider, then they existed as a stand alone range for several years. I wonder if there is any documentation on these nibs anywhere?
Just speculating, but, is it possible that the missing numbered nibs could be broads or special nibs? As I have not seen (not yet at least) any example of a broad version of these nibs, medium yes, but no larger.
The un numbered sliding nibs are also of interest as I am curious to know if they had much size variation and how this was this determined.
On another note, it appears that the level of flex in the range did vary as the nibs I have display slightly different levels of flexibility and this may have to do with the shape of the nib, which does vary across the range as well as the thickness of the nib material itself. For instance, the most flexible nib in the range I have is a 3 fine. So, smallest of the range but thinner and less metal to bend so more flexible perhaps.