This final part of the trilogy attempts to cement the motivation behind the Zodiac Killer's crosshairs he gave us on the Phillips 66 Road Map as part of the Button Letter mailed on June 26th 1970 and the Little List Letter one month later on July 26th 1970. In the following diagram we have incorporated the three crosshairs together to better explain the findings. This idea has been covered before, but finalizes the three part story under one banner in a little more detail.
However this would not have been possible using the Button Letter and Phillips 66 Road Map alone. The Zodiac realizing this gave us the two additional crosshairs on the Little List Letter, with a couple more hints. Firstly the bold circle and secondly the phrase 'PS The Mt Diablo code concerns Radians + inches along the radians'. The Zodiac Killer however placed this phrase over the crosshair itself, when he could have easily accommodated both elements separately, unless he had an ulterior motive.
The phrase runs over the lower north-south axis; to be precise over the 180 degree axis or number 6 on the Zodiac dial. In fact only the word Diablo is bisected by this line.
Let us expand upon the 'Map coupled with this code will tell you where the bomb is set' to better explain things, using this phrase 'The Phillips 66 Map coupled with this code will tell you where the bomb is set'.
In the first instance the code of San Francisco International Airport or 229 degrees was coupled with the map set at Magnetic North of 17 degrees to equal 246 degrees. The Mount Diablo code was written on the 180 degree axis, so we will couple this 180 degrees with the map or more specifically the Phillips 66 Map, to give us 180 + 66 = 246 degrees, the exact position of the bold circle he provided us in the Little List Letter and exactly the same as 4 radians and 17 degrees. Then we shall take the number 6 he provided on the 180 degree axis and once again couple this with the Phillips 66 Map to give us 666, the mark of the Devil or Diablo. The conventional view is that the peak derives its name from the 1805 escape of several Chupcan Native Americans from the Spanish in a nearby willow thicket. The natives seemed to disappear, and the Spanish soldiers thus gave the area the name "Monte del Diablo", meaning "thicket of the devil."
Diablo was placed over the 180 and 6 axis and on both occasions coupled with the 66 from the Phillips 66 Road Map gave us something significant, but whether or not this was his true intention is still unclear and may never be resolved.