Vortex
Vortex has made quite a name for themselves in the field of optics. From their entry level hunting scopes to hard use precision optics in service with US armed forces and high level competitors, they have something for everyone. In addition to the scopes themselves, Vortex supplies all the mounting needs one could have.
Modern rifles such as the AR-15 and precision chassis bolt guns have different mounting systems than the hunting rifles of days gone by. Eye relief and height concerns are where cantilever mounts come into play. Vortex Precision Series mounts are machined to high standards and are available in the required extended style.
Vortex has a quick explanation of cantilever rings.