The seven shell casings were discovered to the right side of the Corvair. To facilitate shooting Michael Mageau in the hip and thigh, it's extremely likely the killer had to lean into the window and over the passenger seat of the Corvair. The top of the passenger seat positioned exactly where Michael Mageau was struck with the first two shots aimed at him. He was struck in the jaw and shoulder, producing 2 entry wounds and 2 exit wounds. Both of these bullets exited his body and struck Darlene Ferrin, who suffered 9 entry wounds in total. With four wounds in total to the upper part of his body at this point, there is a high probability that this created blood transfer to the Corvair's front passenger seat. A passenger seat that Zodiac had to likely lean over in order to aim a gun at Michael Mageau's lower extremities when he took refuge in the back passenger seat area. Very likely on the floorboard itself. This would adequately explain the two stray casings located here.
The nature of the attack, keeping the victims within the vehicle at arms length (unlike Lake Herman Road when he shot David Faraday outside the vehicle at close range through his left ear), seems to suggest that the Zodiac Killer likely never envisioned any close interaction with the victims and the need for a change of clothing during his second attack. However, the Zodiac Killer likely made one crucial error, in returning to the Corvair for a second time when he heard Michael Mageau crying out on the back seat, delivering two more shots into the badly injured young man by leaning into the blood-soaked vehicle. This may have changed his desired plans and ultimately, the timing of the phone call to police dispatcher, Nancy Slover.
Had he not leaned into the vehicle, he could have made the payphone call at approximately 12:10pm before the victims were even discovered. This would have been the much safer option - and the question of the roughly 40 minute delay in making the payphone call and where the Zodiac Killer lived - would never have arisen.