The Lost World Pan edition is 100. There isn't a Son of the Gods Pan edition. Son of the Gods is also unique in that the phrase "A Harlequin-Pan Book" is not on the front cover and no notice of the arrangement with Pan on the copyright page as there is with the other 11 books. I suspect that the variant of Son of the Gods with the Harlequin-Pan notice on the back cover is an error. The Lost World is 238, Son of the Gods 241. Covers with the preprinted phrase used for The Lost World may have been inadvertently used for Son of the Gods before the error was caught. At any rate here are the books.
Harlequin 238 - July 1953
Harlequin 238 back
Harlequin 238 back variant
Harlequin 241 - August 1953
Harlequin 241 back
Harlequin 241 back variant