Patrick--I think it is amazing you are lacking just one gray back. Hope you come across Mr Reiser soon.
Have you looked at the H&S Super set checklist ? There is no doubt some of the listed variants are just printing flaws, but they are recurring and I was able to run them down fairly easily.
As I recall very few of the print defect variants were in the high series, although Campanella is one. Maybe no one wants to find any there
The 3 high series DPs are on the list