The B4/C4 flash (cube) adapter for Minox B was initially designed and used the Mallory PX825, cell battery. These are now extinct, because they contained mercury oxide. This very rare coin type cell was once commonplace and used to power Minox C4 flash cubes (requires two), Vitessa 500AE cameras (Zeiss Ikon Voigtldnder Vitessa 500 AE), Agfa Rapid cameras, Agfa Agfamatic 126 cameras, Kodak Instamatic Reflex cameras and many others. Kodak Instamatic 124 and the Kodak Instamatic 714 take this PX825 battery. Kodak also made thousands of collapsible Instamatic cameras called the S10 and S20, which took two 825's side by side.
The PX825 is a 1.55 Volt Silver Oxide button cell that, until very recently, was no longer manufactured.
Type: Silver Oxide
Output Voltage: 1.55 V
Chemistry: Silver Oxide
Diameter: 0.91in (23.2 mm)
Height: 0.24in (6.1 mm)
This type of battery os also known under these designations:
LR53, EPX825, V825PX, HPX825, RPX825, BLR53, KA825, Pertrix 7201. NEDA designation is 1129AP
You can check availability here.