The numbering is a bit of mystery that will probably never be solved. Here's a few more cool ones. #2 is the first fiction title...
#7 is the last Red Circle before it changed to Lion Books...
#8 is the first Lion and one of four Lions before they changed the imprint back to Red Circle for #12 & 13 ("Carnival of Love", above is the last Red Circle)
With #14, Shirley Jackson's classic horror collection "The Lottery", Goodman changed it back to Lion for the rest of the run. I love my Avons, Pops and Pyramids, but if there's one manageable run I'd try to complete, it would be the Lions. All the great 1st editions of Thompson, Matheson, Bloch etc and great covers by the entirety of Goodmans Mens Adventure Mag artist stable