I would say electrical tape seemed to be used on occasion early on as a handle wrap. There are so few from the early period that you just don't see enough of them to really claim anything was used with regularity. Most of the very early ones are just straight handles with nothing on them.

I believe as the model became more popular during the Viet Nam war you began to see different types of wrapping material used and more frequently. I saw one once that had a bicycle handle bar grip on it. Basically an oversized crutch-tip. Actually worked pretty well as a grip but was a pain to get off the tube.