I agree with Chopper, I think. A chopper lump has to be integral to the barrel forging, correct? So each barrel has half the lug, and in the case of the 21, there are surfaces milled into each demi lump which mate mechanically somewhat in the manner of a "dovetail" but have neither a full "pin" nor full "tail" as in dovetailed furniture or the "dovetail" ways of machine tools (probably should be called a "wedgelok").

jack