My original tumblr post was here. This image was researched by Bacchus at ErosBlog as part of the “Δ commission.” The research was originally published at Hedonix as “Δ 057 – Tube Twins.” Here is what Bacchus found.

This image is a cutout detail (heavily blown up) from the cover of a modern edition of Twin Of The Amazon, a 1948 pulp novel from the 27-volume Golden Amazon series, by author John Russell Fearn:

According to the International Science Fiction Database, the full cover art shown directly above is by artist Ron Turner and was scanned from a 1998 re-release edition of the book. Turner’s career credits begin in 1950 and continued through his death in 1998, so it is clear that this cover could not have appeared on the original 1948 edition of the book. However, both Wikipedia and the ISFDB indicate there was also a 1954 edition. The Wikipedia entry for Ron Turner, though, suggests he did not begin drawing cover art for Gryphon Books until the 1990s, leading to the conclusion that this artwork is quite modern. Indeed, the cover of one 1954 edition seen here is far less lurid: