manwithamoviecamera

I have the same issue with an approx. 50-year-old Japanese photographic slide viewer that I bought today, Dave, and which also was designed to take (a pair of) UM-3 batteries. According to Wikipedia, "An AA cell measures 49.2–50.5 mm (1.94–1.99 in) in length, including the button terminal—and 13.5–14.5 mm (0.53–0.57 in) in diameter." My guess is that such batteries were originally manufactured to be nearer the 49.2 mm end of said size range, and over the years have grown such that a new "AA" battery is closer to the 50.5 mm end? (Think I'll try filing half a mm off of the positive terminal! Worse, I also have old equipment that takes four AA batteries, in which some modern, high capacity, rechargeable are slightly too fat to fit!)