You might want to check out this Leg Strap Adapter for the 3-screw Blackhawk mounting system... "The Blackhawk T-Series Jacket Slot Leg Strap Adapter has a triangular shaped adapter that tapers up to a single pivot point, which mounts using the bottom screw hole of the existing 3 screw Blackhawk mounting system. The adapter affixes to the Jacket Slot system via a single aluminum bushing, through the existing bottom screw hole.
Lever action shotguns are fun to use for us but not exactly popular for many shotgun users nowadays. Here is one that has a good history for The Firearm Blog to feature in their TFBTV channel, the Chipappa 1887 T-Series... "Lever action shotguns were not popular for a long period of time, and even John Browning suggest that Winchester opt for a pump action in lieu of a lever design. However employer trumped employee and thus the 1887 was born.