ntxt

All Superposed models were made in Belgium.

I can't tell you the years, but the flat-bottomed pistol grip was used on the Belgian Superposed shotguns during most of the 1970's (at least) and reverted to the round knob sometime later.

Actually, the flat knob didn't come out till 66'-67'.
The round knob was used before that.
I've seen some round knob's on high grade guns post 67' and I suppose you could special order a round knob on any gun.
I see your looking for a 20 ga.
Good luck, they come dear. I have a 1949 20 ga. I like the early 20's. They came with a slimer stock and forend.

The "Round Knob Long Tang" made a resurgence in the early 80s. All B-25 presentation guns were made RKLT / Straight grip...In fact, thae majority of FN custom shop guns have been made RKLT in the past 25 years.

The "Flat knob" guns ran from about 65-66 - through 70s.

However when the B-25 (Superposed) went into "Custom Shop" only production, Round knob guns custom ordered and the stock "Classic" models became the norm - I've owned a ton of post 1980 guns with RKLT.

Important to note that up to this day, you can order a Superposed any way you like it!

Just a little FYI.