Camelion 4SR44 or Varta V28PX PX28 6V Silver Oxide battery.
Camelion 4SR44 6V Silver Oxide Battery is a popular silver-oxide cell commonly used in cameras (such as the Minox 35ML, Bronica GS-1, and Canon AE-1), light meters, and blood sugar testers.
Also, a replacement for the now discontinued PX28 mercury battery.