William V. Peekaboo Pennington Senior was a San Francisco private detective, described as a cattle man, private investigator, theater owner, building contractor, oil man and member of the Eastern Star, a freemasonry related order established in 1850 by Rob Morris. He moved to San Francisco in 1924 and performed investigative duties for prominent lawyers and attorneys, including Melvin Belli, who became closely embroiled in the Zodiac Killer case.
Clearly William Pennington is not the Zodiac Killer, as the timeline crossover is inadequate, however, we have seen the 'cattle connection' previously with Fred Harman, the Red Ryder comic strip, the strange unidentified logo displayed on the Halloween card envelope and the 'skeleton card' inner. This "peek-a boo" imagery is not conclusive, but nonetheless is very interesting when we search for the inspiration behind the Zodiac Killer and his many communications. Here are the appropriate links to read the threads on ZodiacKillerSite forum and ZodiacKiller.com forum.
Location of 210 Post Street on Google Maps.