There are a couple of patterns to note. All four books are from the first group published in 1940 (numbers 1 to 20). The back cover ad on the three below is the earliest version seen only on the first group of Bantams. No conclusions from this small sample but as I find more we'll see if they hold up.
Bantam 9
Bantam 9 back
Bantam 17
Bantam 20