Posted By:
RichardWow - a 2 and a half year old thread dug up, and I actually started it. I feel dated.
From what I have seen over the years, the pop reports by both SGC and PSA are unreliable for these issues because:
1) they incorrectly categorize the issues some of the time (ie. Blank backed are called Collins McCarthy or W575)
2) a person or several people have submitted large portions of certain sets which have skewed a true sense of rarity
3) the numbers for most of these sets are so small that no detailed conclusions can be made
There was also mention that the 1917 Standard Biscuit oftens gets mixed up with the 1921 version. I would also agree with that.
That being said, based on my experience I feel that Boston Store is the most common (which is still very very uncommon) followed closely by Collins McCarthy.
Then there is a huge jump down to Standard Biscuit followed by Weil and Blank Back.
Something like this (10 being most common and 1 being least common)
Boston Store 10
Collins Mccarthy 9
Standard Biscuit 3
Weil 2
Blank Back 2
Based on this scale, something like E121 would be a 40 or 50.