You could be right about Sturgis and the 60's, I'm not real sure either, but...
The most famous (in-famous?) hill climb event in the US is The Widow Maker in Utah. I tried to do some quick on line research and came up with some bogus facts. Wikipedia claimed the event has only been running for the past 6 years, which is simply not so.
In the 70's cult film "On Any Sunday" which was made in '71, Bruce Brown visited and filmed the Widow Maker and gave it quite a bit of coverage in the film. Even then there were comments made about "longest running", "historic", etc. I'm sure it was said how long it had been going on, but I don't recall. '71 was the first year that anyone ever made it to the top too. I don't remember the riders name but he was riding a stretched, nitro injected Harley powered purpose built machine and that too is in the movie.
With suspension technology advancing as it has been for so many years the sport literally turned into a drag race up a hill. So much so that several years ago Pro Circuit built a bike using two Honda CR500 engines fueled by a methanol. It's my understanding that the twin engine bikes were outlawed along with any fuel except gasoline.
I've seen some modern day hill climbs on TV, the most recent was an event held in California at the Glen Helen Raceway. Interesting stuff, the hill wasn't terribly long but extremely technical. Most of the participants were making it to the top so timing was the tie breaker.
Bruce