I started working on a backs run way back, but never came across some of them and hadn't considered factories at the time. Still got pretty close, missing all the usual ones. Oddly I missed a SC150 overprint but fixed that last year. Only one left that's within budget for a rough example
P350-460fact 42
Drum
Uzit
BL 460
OM brown
OM blue - Just had to add it
Lenox brown
Hindu red
Cobb
(And T213-1, T215 etc if you count them)
On the plus side I bought most of them way before backs were a major thing, so it was pretty inexpensive in low grades.
Steve B