Probstein, two things.

[BigJJ]

Such backs are more rare than other, more plentiful, backs. Believe the description is fair. In general, and in the business terminology, it passes use of the word "rare".

Perhaps in the industry we should have a common definition of "rare" - for pre-war, under 1,000? examples - for post-war, under 10,000?. and then perhaps a common definition of "ultra scarce" - for pre-war, under 100? - for post-war, under 1,000? and then perhaps a "handful" for pre-war under 10, and post-war under 100? Trying to assign numerical value to terms I think would be an interesting idea.

Becomes a little more interesting with T-206 of course as a Frank Chance T-206 may not be rare, but one with a particular back might be.

Ideas/views with regard to setting terms for number of examples that survive?

I started a new thread along these lines. Not Probsteins fault we do not have any guidelines really. Im cracking up.

Best,
J