Well, I spent a lot of time pushing a cart through the commissary on Aberdeen Proving Ground in the 1980s. We had the old, manual NCR cash registers.
The first time I ever saw a scanner in actual use in a grocery store was in a Wegmans supermarket in Liverpool, NY. That would have to have been before or during 1978, because that's the year I left central New York for my first job in North Carolina.
It didn't really come as too much of a surprise because IBM had been running commercials on the Sunday-morning news magazine shows demonstrating the technology
The Wikipedia article on the Universal Product Code contains this statement:
"The first UPC marked item ever scanned at a retail checkout was at the Marsh supermarket in Troy, Ohio at 8:01 a.m. on June 26, 1974, and was a 10-pack (50 sticks) of Wrigley's Juicy Fruit chewing gum"