1958 !
Well done Jedgerley, the closest so far to the 1956 "Untamed" !!
Unbeaten, the poster for "Untamed" (1956) remains the first ever between-the-legs-shot poster unless...
(note I have done hotlinking for images but I can see it is possible to attach pics in APF, so I shall download image and repost previous posts with attachment)