Watch Battery, Button Cell Battery, 42mAh, 1.6V
From Amazon
VARTA Batteries Electronics V391 button cell battery 1-pack, Button cells in original blister pack of 1
BatteryUpgrade
Varta V391 / SR55 / 381 Coin cell battery (Amount 1)
Conrad Electronic
Varta Button cell SR55, SR1121 1.55 V 1 pc(s) 42 mAh Silver oxide SILVER Coin V391/SR55 NaBli 1