I did not need correcting, Steve. I was correct - it was not WIDELY used until the 1970s, regardless of what Wikipedia says about the incidental uses up until the 1970s. Before that, the term was specific to a few users. Until 1965 it was a flat red line. By the late 1970s, it gained traction. Again, look at the Ngram viewer. That is Google's scan of every book in existence and looking at the use of individual words. The word with the capital H was not in common usage in that form until much later than the Nazi genocide.

That Wikipedia page is probably maintained by the Holocaust Museum library staff, who have no interest or need in defining when the word went into common usage for their purposes. My university library maintains Wikipedia pages on the topics where we hold in important collections and we do try to influence what things are called (for example the "Mexican-American War" is actually most correctly termed the "US-Mexico War" because "American" can mean all of the New World, not just the US in particular). Clearly some readers here encountered capital H Holocaust earlier than others. It started being used in publications in the mid-1960s, barely a blip, but it rises in use through the 70s. The case can be made that the book gillymor mentioned was the published start of that usage.