To improve the interference immunity of Perst-4, we upgraded the electronic block. That required changes of the electronics of the tactical switch. The update did not change tactical features of the device, but will be important for users, who plan to operate it simultaneously with Klesch-2 dual flashlight. If your Perst-4 was made before an update, you will need KV-4/2 tactical switch. If it was made after an update - KV-4/2U (universal). To avoid confusion we made new graving on the body of the device “КВ-5ПУ”. It is placed under an connector. If you don’t have it - you will need KV-4/2. If you have it - KV-4/2U. More details - in new video on “Zenitco in English” YouTube channel.