Hi Jason:
Sorry I didn't see this post right away. I collect Johnson Confectioners cards, so I do know a little bit about them.
I'm glad you got the Scott - it's a great-looking example and the price was outstanding. I didn't bid because I already have one.
In terms of rarity, it's really tough to tell. Finding a card with the back stamp for a type collection isn't particularly tough - they come up from time to time if you look hard enough, and occasionally you'll find them in larger lots of E121s (I've found a few of them that way).
At the same time, they ARE pretty tough - it's taken me three years and change to get the ones I have. Looking for a specific player or team is even harder.
Because it's the only set I collect with a specific back, I wouldn't know how to compare it to some of the better-known tough backs. I'd say it's up there in terms of rarity, but nowhere close in terms of demand. Most of the tougher T206 backs, for example, seem easier to find, but sell for much higher prices because more people want them. E121s and their ilk don't seem to have as many people collecting them, and since most of the Henry Johnsons are beaters, they don't really attract the high-grade crowd.
Either way, it's a great pickup at $20 for a card that nice. I've generally been paying $20 for beat-up commons; Scott is a little better than a common and that example you had is very nice (nicer than mine).
Great job!
-Al
PS: Here's my favorite card from the set so far:
