The Saalfield reprints
These reprints are from around 1935-1940. They came in 2 heights, and used the same artwork for all three books printed. The covers were also printed in several colors.
Saalfield boxed set
This is a truly scarce, if not rare, set. But almost no demand (if any) for it. Even I'm not really interested in buying it! =) Saalfield released several series' in boxed sets, and they are all rather difficult to find. These pictures came from an eBay auction.
Later Saalfield reprints
Saalfield reprinted just the first book in this format, in both hardcover and paperback.