B25 = Superposed = B25. ALL of the Superposed/B25 guns were and are, made by FN in Belgium. The Browning Superposed was offered by Browning as a standard product beginning in the mid 1930s. It was discontinued as a 'standard' product and replaced by the Citori in the 1970's (perhaps early 80s) and become a special order item from Browning/FN thereafter.
The Superposed was designed by John Browning and brought to fruition by Val Browning. It was very popular in the USA and came to be recognized as a fine firearm worldwide. There are many variations in this design, as you might expect for a firearm that has been produced for 90 years.
The Superposed/B25 was originally available only in 12 gauge, but in 1949 Browning began offering a 20 gauge model. Since then they have also been offered in 28 gauge and .410.
I am very fond of the Superposed 20 gauge guns, have owned more than a dozen, and currently have a 1937 12 gauge and a 1950s 20 gauge - both with solid ribs and 28" barrels. IMO, the guns made from the 1940s to the late 1950s are the best in terms of build quality, dynamics, and overall 'shootability'.
A good Superposed 12 can be had in the USA for about $1500 and a 20 gauge for $3000 and up in the lower grades (depending on condition, barrel length, vintage, grade, choking, etc.). Ordering a new B25 from FN is considerably more, although they are very fine guns.