A few things came to mind (hey Johnny
) when I read the title of the thread.
Some people do a back run of a certain player-in that case you'd have to know for a fact there was an upsidedown back for that player.
Some people's back run will simply consist of one of each tobacco brand. Some people take it a little further and want an example of each factory from each tobacco brand.
Then you have people who love the freaks
and of course, that's where an upsidedown back fits into the picture. If you collect these types of cards, then I see no reason you can't create a list (as Johnny did) of scarcity- IMO EVERY freak card is a rarity in itself- many only one to a handful known- and the ones who collect these cards would be the ones to know the scarcity and the ones able to rank the difficulty.
I guess my long winded point was to say it depends on what "type" of back run one is doing because not everyone does the same type of back run.
Cool thread, I love the topic.
Sincerely, Clayton