30" 12 gauges are plentiful but there never was a 30" 20 gauge. The long tang / short tang represented a time (1967) when Browning was beginning to feel the pinch.
Things were getting expensive, especially skilled Belgian labor and the tang and knob changes represented cost savings. Some will say the earlier guns were of better quality.
It wasn't too many years later when Browning moved Superposed production to a custom shop status and began the Citori line (a Superposed knock-off) in Japan.