bonus content. This chart here shows the cards where the 3 scarcest backs all have a combined PSA/SGC population of 3 or less.
I would say these cards would be insanely difficult to complete back runs for
And if my numbers are right, the hardest back run of them all is technically Doyle With Bat, that has 3 pop 1s (BL 460, Drum, Hindu Red). The BL 460 shows up as possible on T206resource. SGC shows an SGC 1 copy graded.
That is...tough.