This dust jacket format included paper dolls for Freddie and Flossie on the back cover.  As far as I've seen, Nan and Bert were never included.  The copies I have list to titles from Wonderful Secret to Radio Play on the front flap, showing that these were printed from around 1931-1937. 

I don't know how many total were printed in this format.  I believe at least 1-30 were printed thus.  Most of the pictures below are my scans, but a few are taken from eBay auctions. 

They are not scarce at all, contrary to what some sellers might say, since they were in print for
at least approximately 6 years. 

Although the majority of these were printed with the green cover below, with the circle showing the sets of twins, I do have one that has the "Paper Doll dustjacket" with the "Thelma Gooch Applique Cover".  This is to be expected with especially early or late printings of any format, as dust jackets and books could be mixed between printings.

---

#1

#2

#3

#4

#5

#6

#7

#8

#9

#10

#11

#12

#13

#14

#15

#16

#17

#18

#19

#20

#21

22

#23

#24

#25

#26

#27

#28

#29

#30