A member on the Zodiac Killer Site forum going by the pseudonym Blind Bat has made a very interesting connection to the Halloween card, involving the "peek-a-boo" phrase the author added to the greeting card. The picture on the left demonstrates this perfectly with the Zodiac-like letter K and the eyes and eyelashes eerily similar to the depiction on the Halloween vard itself.

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.
               
Zodiac Killer Net
http://zodiackiller.fr.yuku.com/topic/7412/William-PeekABoo-Pennington-SF-PI#.VMI79UfgHLW
Location of 210 Post Street on Google Maps.

"Would the writing on the car door be enough to convict Zodiac had he been caught for one of the other Zodiac murders"? Asks Tahoe27. The topic then expands into all aspects, including handwriting analysis, the validity of this attack being placed under the umbrella of Zodiac murders, the specifics of the crime and the phone call from 1231 Main Street, Napa, to name but a few.
The Lake Berryessa crime, the third in the series, showed a marked deviation from the two previous attacks at Lake Herman Road and Blue Rock Springs, in that the attack displayed a flair for the dramatic, when the supposed Zodiac Killer donned a black executioner's hood and a waistline bib. However, despite his theatrical entrance, this was soon to turn deadly.
To view this thread: Zodiac Killer Net

Three young women, students of Pacific Union College had traveled to Lake Berryessa for a spot of sunbathing, parking their vehicle two miles north of the A & W Root Beer stand on Knoxville Road. After they had exited the car, a white male, driving a silver or light-blue Chevrolet 2-door sedan with California plates, pulled up to the rear of their car but remained inside his vehicle. Thirty minutes had elapsed before they noticed the same man, approximately 50 feet away, observing them sunbathing - and each time they looked at him he evaded eye contact. He was described as 28 to 40 years of age, 200 to 210 pounds in weight, 6 feet tall, styled black hair, with rounded eyes and thin lips. The young women added that he was good looking, with a muscular or stocky build, wearing dark pants and a dark pullover shirt. The composite sketch of the man drawn from the women's recollections that day is shown to the left.
The Zodiac Killer Net forum thread, started by Tahoe27 can be accessed here 

In Welsh Chappie's own words "I am starting this thread because this issue is one that seems to get overlooked more often than not. I realize that this may be due, in part, to the source of this claim being 'The Yellow Book', but as far as i have been able to determine, there is a police report that states these calls were made. So, based on this, let's look at the events of that night and what the calls could mean, and reasonable assumption one can make regarding them". 

Questions have always remained over these phone calls, so to investigate this topic further and possibly contribute, visit the thread at ZodiacKillerNet

Richard Gaikowski (March 14th 1936 - April 30th 2004), was a newspaper journalist and filmmaker, who also became linked to the Zodiac case in respect to several key points, having been brought to the attention of Zodiac investigators by an informant, who to protect his identity went by the codename Goldcatcher and worked with Gaikowski at a Bay Area newspaper, 

Morf13 started a forum thread examining the handwriting of  Zodiac suspect Richard Gaikowski compared to known Zodiac samples, as well further analysis of letters composed by Richard Gaikowski, while examining his perceived whereabouts in the Zodiac Killer timeline. He is the favorite suspect of Tom Voigt from Zodiackiller.com, who himself provides contribution to this topic. Visit the point of discussion here: 
Zodiac Killer Net

The latest Zodiac suspect has just been revealed as Earl Van Best Jr, amid a flurry of newspaper and television articles and claimed in a book by Gary Stewart that his father was indeed the infamous Northern California killer. The book has been released by Harper Collins entitled The Most Dangerous Animal of All.

However, police seemed to know little, until the publication hit the bookshelves and we will have to wait on the validity of this latest claim. To purchase this book and decide for yourselves, it can be obtained here.

In what was dubbed 'The Ice Cream Romance', Earl Van Best Jr appeared in court on a KPIX news report from August 31st 1962, charged for the offence of eloping with a minor, Judy Chandler, aged 13. See video.

'Seagull' opened up a discussion thread on this topic on May 12th 2014, containing all the relevant news links from around the internet. If you would like to join the conversation follow this link to the ZodiacKillerNet forum thread. To visit The Most Dangerous Animal of All website click here.