This is a real billboard from 1970, found by the Public Collectors .
The west coast chain has long been known for their sexual adverts (one recent TV example, below). So, I don't doubt that this butt-licking joke board could be real. If you lived in Southern California at the time, and remember seeing it, please comment. UPDATE: a FB commenter points out that "ring job" is an auto service term. Still, I believe the double entendre was intentional.
Source: publiccollectors