Re: supplemental ground rod
You are close Tom, I don't know the exact time.
The first time I used PVC was in 1963. The job was in the Marshall Islands.
The wild pigs liked the glue, and became glue sniffers. They rooted up our 4160 volt lines and bit into them, once.
I965 NEC, Section 250-81, stipulates the supplementary electrode is required. The reason is when the length buried is not confirmed and the chance of plastic being used.