Looking at the car door from Lake Berryessa on 9/27/69. I'm thinking that Zodiac did that as an update for the number games I believe he was playing.

12/20/68=100

7/4/69=80

9/27/69=105

So, with 9/27/69 he's continuing using the multiples of five dates but also he has gone around a 360 degree circle:

12/20/68 Vallejo (Vallejo(77)+Zodiac(58)=135) (numbers-for-letters)

7/4/69 Vallejo (Vallejo(77)+Zodiac(58)=135) (numbers-for-letters)

135 +135=270

9/27/69 Napa (Napa(32)+Zodiac(58)=90) (numbers-for-letters)

135+135+90=360 (360 degrees in a circle)

And it's the perfect date to allude to the completion of a rotation of a circle with 9/27/69 because the 4 points on a 360 degree circle are 90,180,270,360

Take off the zero and you have 9,18,27,36

So, with a date of 9/27 that's two points:

(9)

(27)

9+27=(36)

27-9=(18)

And then the mention of a 6:30 time. 630 is a number anagram for 360.