It's job #225 in the log, which shows that Dell commissioned him on February 7, 1962, and paid him $200 on April 17. The original painting - wherever it is! - is most likely gouache on illustration board.
This one's closer to his usual fare than the Monster Parade covers, but it's still an oddball - at least compared to much of his other science fiction pictures. Its "retro" quality would make it at home on a pulp magazine or B-movie poster of the 1940s or 1950s. He sure could could lay it on thick, when need be.