If you have a sight, you may need this!
The CR1620 battery is a slightly less common battery than the CR2032, but it is used in many smaller red dot sights to power the illumination of the reticle. This type of battery is compact and lightweight, making it ideal for use in small electronic devices. It has a voltage of 3V. Some sights use this battery in order for it to be powered. Please make sure your sight uses a CR1620 before purchasing.The usual sights that will use this are RMR pistol red dots and T1 style red dots. Many other sights may use this battery as well.
(Packaging and brand may vary, but the battery will be in some sort of packaging)