I was curious, so I did a bit more reading on it. They started standardizing all the lines to 120 at the supply point in the 1930's, since they were getting some noticeable loss over wire distances. They wanted to make sure the end user had 115 by the time it got there.
It makes sense that old timers in 1975 would have known nothing but 120, since if they had been fooling with it for 40 years, it would have still been after the change to 120.
Guys like Mr. Edwin Williams, that I worked for, and my Dad probably started fooling with it in the '20s, when they were probably lucky if they had 110 volts.
Just a reminder for anyone wanting to order the Paolini pocket rule OTT, you have until Monday. I just ordered the Stainless steel 6", 8" and 12" in metric/imperial combo format. Hope its here by Christmas so my wife can put it under the tree for me