From the Paxman website:
Richard Merewether’s association with Paxman started with alto horns first a single horn in f-alto, then the first dual-bore F/f-alto double horn, and subsequently a Bb(A-+)/f-alto. This was an important breakthrough in horn design. Since then Paxman descants have been regarded as the benchmark for this kind of horn.
All of our descant horns incorporate a unique valve, designed by Paxman, that gives the airway diverging routes from a point close to the mouthpiece where the tube diameter is still small. This allows both the Bb and f-alto sides to have the correct tapers for both the mouthpipe and branch. The two sides then converge in the same valve, in another chamber with a much larger tube diameter, in the expanding approach to the bell throat.
Paxman’s Model 40 Bb(A-+)/f-alto was introduced in 1986. The latest development is the Model 45, which is essentially the same instrument but with narrower cylindrical tubing on the f-alto horn, which enhances the sound quality and equalises the resistance of the two sides. The Model 60 is a Bb(A-+)/bb-soprano horn.