The "spud" that you are referencing is in fact a trench gun part as I assume your butt stock is as well. It is there to position and retain your magazine tube using the bayonet lug. As a matter of fact, remove the clamp and see if the barrel has the "positioning" boss on it or if it is smooth. If the barrel is smooth underneath, you actually have a trench gun sans the heat shield and bayonet lug. I have never seen sling swivels on riot guns. But I have certainly not seen them all either. I have seen trench guns without the bayonet lug that had their magazine tubes held in place with the clamp pictured here. It is originally off of a Model 50 Winchester......



Cheers