870 16's have always been made on the 12 gauge frame. The difference is in the barrels. The early guns were trim and light without vent ribs, etc. Later guns, the mandate of steel shot and economics created the pigs that are late model 870 16 gauges. The new barrels are made from 12 gauge blanks, so are thicker and heavier, add a vent rib and the weight increases again.
