Origin of elbow sex

[bjjohns]

OK, so I am watching Young Frankenstein this evening, and I see a elbow sex scene. Got thinking about it, and I know elbow sex became a popular concept in the Rocky Horror Picture show.

I ambled off to my computer to try and determine the origin of elbow sex. Couldn't find much. SO I started looking at movie dates. YF was 1974, RHPS was 1975. Anyone got one sooner?