Base mount weaver

Height increasing base is used to adjust sighting line position when red-dot in mounted at the front part of carbine. 

In practice it used in versatile sighting system (red-dot plus magnifier).

Example of use with AKM. At this carbine you can see gas tube with Weaver rail and when red-dot is installed on it appears that optical magnifier is higher. In this case you can use base mount (Weaver-Weaver adapter) to adjust scope height.