Hi Brad, I believe the order of scarcity for the backs are as follows:
1927 Copyright
Ad Back
Stats Back (or sometimes labeled 1926 Copyright)
With the Stats back being the most common. I believe the thinking is that the blank back version was cut down from an uncut sheet (
Link). Not sure how obvious that is on your version. The scarcity of the blank back versions are probably on par with the 1927 Copyright backs.