Assuming that the 2cm mass by the gonial angle of the mandible, and 0.5 inch bruise on the left side of the neck were not resultant from an action prior to them arriving at the Lake Herman Road turnout, what interaction with the killer could have caused these two features described at autopsy.
The first thing I considered was a killer standing in front of David, holding him around the neck with his left hand, and pressing his thumb into the left side of David's neck, thereby causing the 0.5 inch bruise. The second possibility was a killer forcefully pressing the gun into David's neck, before switching position to fire the fatal shot into the "upper slight posterior area of the left ear". Had such an action transpired, it would suggest that David was held by the assailant for a period of time prior to his ultimate murder. A shot from this position could account for the shell casing found on the front passenger floorboard of the Rambler at the crime scene - which may never have arrived at the laboratory for analysis - and therefore omitted from the Department of Justice report.
The crucial part of the Department of Justice report is the statement that ''All bullets submitted were Western copper coated .22 long rifle bullets, although some were damaged, it was possible to determine all but Item [1] had 6 right hand groove class characteristics". Item [1] was the bullet recovered from David Faraday. The report suggested that because some of the bullets were damaged (such as the one fired into David Faraday), it wasn't possible to determine if this bullet had right hand groove class characteristics like the other bullets fired at the Rambler and Betty Lou Jensen. This leaves the door open that this bullet could have been fired from a second gun. The same could apply to the missing tenth casing from the DOJ report, likely separated from the other nine when the Rambler was towed away. A bullet and casing that may be connected to one another, and a second shooter.
The claim of the author on August 4th 1969 rings somewhat true with regards to Betty Lou Jensen, when the Zodiac Killer stated "All I had to do was spray them as if it was a water hose". However, this claim doesn't apply to David Faraday, who was clinically executed by a close-quarter assailant at point blank range. Did the author of the "Debut of Zodiac" letter overlook the perspective of his accomplice, or was this just another unimportant throwaway comment by the lone Zodiac Killer?
Thanks to Ray Grant for making public the autopsy report.