I've been hearing (e.g. from RamzPaul) that encyclopedias published during the 1950s, 1960s, and the early 1970s make no mention of the Holocaust. I would like to check that claim, preferably without buying a bunch of old encyclopedia sets at $50-$100 per set.

If what I've heard is true, then encyclopedias published prior to some year in the 1970s will not have articles on the Holocaust, but, during one particular year during that decade, lots of articles on the Holocaust will suddenly appear, and thereafter they will be all over the place.

If you have access to an old set of encyclopedias, try to find an article on the Holocaust in it. Tell me whether you do or don't find such an article, the name of the encyclopedia, and the year of publication. Also, if there is an article on the Holocaust, check the narrative in the article and judge whether the story back then is the same as it is now, or whether there have been substantial changes in methods, in events, in numbers, etc.