$11
For a number of devices and sights, a landing base of 11mm was developed, now most of the devices are produced under the weaver rail (21mm) and, accordingly, new carbines are supplied with a weaver. But if there is a time-tested sight with a landing base of 11mm, then you will need an adapter to install it. The adapter is equipped with two screws with retainers that securely hold it on the weaver.