Aftermarket headset lockrings were a solution to the problem of regular threaded headsets on mountain bikes coming loose on the trail. Before then, it was a case of tightening the locknuts together even harder together, which often caused unsightly damage/indentations to the locknuts.
The original aftermarket locknut was the Gorilla Headlock which, when threaded onto the fork steerer, could then be clamped in place via an allen key. Kona/Joe Murray took this idea a step further in 1991 by introducing the Kona Impact Headset which featured an integrated clamping locknut.
Acor
Acor Frankie
FSA
FSA SureLock
Gorilla
Gorilla Headlock ‘Standard’
Gorilla Headlock ‘Ultralight’
Grey Components (ZooBits)
Grey Components Threadlock
Tien Hsin
Tien Hsin Double Lock
Wheels Manufacturing
Wheels Manufacturing Growler Headlock