Grosset & Dunlap/Chatterton Peck hybrid
This is truly about the rarest of the rare. Two are known to exist. It could be considered the 1st Grosset & Dunlap printing, but only because the cover is stamped Grosset & Dunlap on the bottom of the spine. Before G&D began publishing the series in the first format, they purchased some unbound interiors from Chatterton Peck, who had printed the series before G&D. They printed new covers in the original Chatterton Peck format, but with Grosset & Dunlap on the spine, and put them on the Chatterton Peck interiors. So the cover is G&D and the interior is CP.