Rocketman, with all due respect - I'm don't know what the numbers mean other than LOP, barrel length, and weight and don't think I want to know anymore. All I care about is how it feels when mounting and swinging. I own Parkers with # 1 , 1 1/2 , 2 , and 4 frames - they all swing great - the 10ga #4 frame has very thick barrels and is quite heavy. Paul