The picture should be from 1966, since the zoo was founded on October 2nd 1916.
I used to have that job! It was the late 90's and the truck were way different!
The registration stickers appear to be White and Yellow, that would be 64 and 66, someone may have put the 66 over the 65 that would have been orange. Typically people alternated them, I don't think there was a requirement to do so.