I believe the double boehm clarinets were made by Leblanc as the inventor worked for them.
Excerpt from "The Clarinet": "A fingering system patented in 1948 by Charles Houvenaghel (1878-1966), of Leblanc. In this system the lower joint is that of a normal Boehm system clarinet. On the upper join there is a plateay key for L1 which, when depressed along with L0, produces Eb4 (rather than E4, as on the normal Boehm system.) This means that the pattern of intervals produced by each hand is the same: a whole step followed by a half step. On the double-Boehm, E4 is produced by L0 and L2. The plateau key is also pierced with a small hole, to facilitate the pitches of the altissimo register. The instrument had refinements of tone hole size and shape and omitted the bridge key between the upper and lower joints."
More info here (in French):
https://clariboles-et-cie.blogspot.com/2009/05/pas-de-panique-de-nouvelles-pages-sur.html