There are points of interest to mention that occur on September 27th at Lake Berryessa, starting first of all with Bryan Hartnell's description of the 12" knife the hooded man used to inflict the horrific injuries or the Two unsuspecting victims. Here is the relevant excerpt from the police report where the description of the knife is mentioned:
Even in todays World with CSI and Forensic Science being as popular as it is, there are seldom examples of people taking such a precaution before repeatedly stabbing someone such as it would appear Zodiac did here. The reason for this is simple, most people don't know and are not aware of the fact that a significantly high number of cases where a victim is stabbed multiple times, the offender will also be injured and not by the victim in their own defence, but by the offenders own knife as the stab the victim because what tends to happen almost without exception is that the victims blood seeps onto the handle of the knife in the offenders hand and thus makes the grip of person holding it very unstable and when they then use force to inflict another stab wound their own hand will slide off the handle and down and onto the blade itself and thus, slicing open the palm of their own hand. One Episode of Homicide Hunter broadcast recently saw Lt. Joe Kandra arrive at a fatal stabbing and follow a trail of blood from the home of the deceased victim and around onto the next street and literally right to the front door of the suspects own home because He had, as a majority do, cut himself in the act of stabbing the victim and bled drops of blood as He fled Home. It is an extremely common injury to occur and yet not widely known by those who end up using a knife to assault others and that's why I say even in todays World, this injury is still extremely commonplace. The offender only discovers this is what will happen after the fact and too late as it leads to their being caught.
So the question needs to be asked: Did Zodiac know this was likely to result from Him stabbing the victims and so add a little gadget to the knife in order to stop His hand slipping off the handle and onto the blade? Certainly appears that way. The subsequent investigation of the area found footprints leading both to and from the crime scene, but no drops of blood along with them so this offender did not suffer this common injury and its likely due to His advanced knowledge of this occurring and adding a support cotton loop to secure His hand and stop it slipping off the blade.
So the obvious question one must ask is, does He know to take this precaution having learned the hard way and cut Himself in a previous outburst? If not, does He work in a setting/job that deals with one or both the victim and/or offenders in stabbing crimes?