It is meant to grip with the right hand and grip the 'D' section with your left hand.
This type of grip is right hand only [I have never seen a left]. This grip is a two handed design [you dont have to use two hands] not a shoulder stock. Barrel length therefore doesnt matter.
Here are some full sized sets. The bottom one is for the octagon. The top one is a Super14. I dont use these much, they look great.