When 1975 came around, the witnesses I knew had heard enough that they thought "no one knows the day or the hour." It is true, however, that 1975 was talked about and speculated on frequently. including the idea that Adam was created on a certain year and the sabbath resting of God would enter into it's last millenium of the 7th 7,000 year long day. (Witnesses used to believe that the creative days in Genesis were 7K years long). But then we were cautioned to realize there could have been several years before Eve was created (and Adam/Eve sinned) so things were still extremely urgent but not necessarily immediate. And this was a loving thing from Jehovah to allow us more time to engage in our lifesaving preaching work.
Only a few that I knew personally in the area actually did highly irrational things. My congregation was made up of a lot of stable older people though, probably already inured to the date setting and disappointments. I was a teen so I was actually relieved when it didn't come then. 'Fraid I would be destroyed for having a few impure thoughts. LOL. I remember telling my non-witness Dad that he needn't bother saving for us to go to college because we wouldn't need it. My first huge mistake.
Even so, we were thinking that Armageddon could come at anytime after that year. This whole thing does quite a number on a person's head. I still have relatives buying this garbled nonsense hook, line and sinker. It's so much easier than facing up to the very real problems humans need to acknowledge and to do something about them. Or facing up to their own mortality.
ETA There are sources that say the year 1975 specifically appeared in 1966 WT and Kingdom Ministries, in talks, etc... http://www.jwfacts.com/watchtower/1975.php but mostly it was alluded to by other phrasing. Maybe the society realizes now that a hint can go a long way when it comes to Witnesses speculating.