This final part of the trilogy will attempt to cement the motivation behind the Zodiac Killer's crosshairs, which 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 Killer 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 placed this phrase over the crosshairs itself, when he could have easily accommodated both elements separately - unless of course he had an ulterior motive.
The phrase "PS The Mt Diablo code concerns Radians + inches along the radians" runs over the south axis; or to be precise, over the 180 degree axis or number 6 on the Zodiac dial. The only word bisected by this south axis is Diablo.
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 to 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 Mount Diablo 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 degree mark and the number 6 axis, and on both occasions coupled with the 66 from the Phillips 66 road map, it gave us something significant. But whether or not this was his true intention is still unclear and may never be resolved.
Footnote: What level of understanding the Zodiac Killer had regarding the concept of radians is unknown, but they are a counterclockwise value around the circumference of a circle. He placed his crosshairs over Mount Diablo, to which he added a sort of clock face, replacing the number twelve with a zero, stating it was "to be set to Mag.N". It can be shown that 4 radians and Magnetic North will fall within the bold, black circle he placed on his 'Little List' crosshairs. In addition, it can be shown that traveling 2 radians counterclockwise from True North will again fall within the bold, black circle. The use of the clock face and the directional north marker set at zero, may indicate that Zodiac intended this as his starting point for any calculations. The mere fact that the above measurements, either clockwise or counterclockwise, using whole radians, both fall inside the black circle, would seem to suggest that the 246 degree angle subtended from Mount Diablo over San Francisco is where we should be looking.