for receivers drilled and tapped for the standard 6-48 screws (most common)

34mm version only available in Medium and High 

heights, measured from the closest contact point at the receiver to the bottom of the ring cradle

extra-low .300", low .400", medium .500", high .600", extra-high .700"

Extended rings:  moves the front ring 1/4" closer to the rear ring than the non-extended version....