It was, in fact, a calculator! Not a pocket calculator, but a machine about the size of a cash register -- four of them were on display. Not only could they add, subtract, multiply, and divide, but they could do even more complicated functions. I was amazed and patiently waited my turn to play with this "computer." And once I sat down to use it, it was hard to pry me off. In retrospect, I think it was probably a Wang 360SE.
It's hard to imagine the days when a calculator that you can now buy at Walgreens for a few dollars was the top draw at a science museum. The pocket calculator revolution arrived amazingly soon after that. And when it arrived, it was more sudden and more complete than even the personal computer/Internet revolution. In my sophomore year of high school (1974-75), we all had to learn to use a slide rule for our physics class. By the next year, everyone in the school owned a calculator. Of course, it wasn't long before most people forgot how to do arithmetic, just as Isaac Asimov predicted in this short story, "The Feeling of Power."