Early 21" & Super 16 barrels had only one lug and the period forearms of that time only had one screw hole. Some where around maybe 1989 to 1991 they changed to two lugs on those barrels, my memory is a little fuzzy on those dates. A two hole carbine forend will match the one mounting hole perfectly, the other stays open. Ed usually has one hole forends for sale if you would like a one hole to match the barrel.