The MOI Garwood measured and reported was the MOI around the center of balance. That is, the gun was rotated around the center of balance to measure the MOIs he reported. Garwood's MOI measured the rotational inertia of a gun about that gun's center of gravity. Had Garwood measured the MOI about the buttplate that would also give an MOI number but different than the one about the COB.
But in the context he said it in, after he had measured and tabulated the values you list of many British game guns, he noted that those with the balance in that range felt "lively". Not that the COB made it so, just that COB in that range tended to coincide with guns he deemed lively.
I believe I understand both Garwood's writings and the physics behind his MOI measurements. I believe I expended the necessary effort.
I think my original post is accurate. I think it is a reasonable answer to the question that was asked.
