Bullet drop compensator for leupold LR scopes

[Ridge Runner]

I saw one in a video, the one used in beyond belief, looked like an M4 turret cap but was marked in yards, anybody know who makes them?
RR

 

[sig man]

http://www.kentonindustries.com/

 

[Ridge Runner]

Anybody used these, I looked them over and still have more research to do, I saw the one John Burns uses and it looks just like an m1 turret knob, I emailed him asking who manufactured it but have gotten no reply
JS