I finally bought a Couesnon. This one has to be from 1930 or before, probably 1920 or before. This is not the model that is marked Monopole. (See below on that.)
Bore: 14.65 at top of left hand joint, 14.8 at the bottom of the same joint.
Special feature: This instrument has the ring for the third finger left hand, used for the forked E-flat/B-flat. The left hand chromatic E-flat/B-flat key also closes a little extra pad, and can be used to make the high C in tune.
Serial: none in normal places. In two hidden places under the keys of the left and right hand joints: X18.
T is stamped on the back of the left F#/C# key.
The stamp on the bell says
EXPOSITION 1900 DE PARIS
MEMBRE DU JURY
COUESNON & Cie
84 RUE D'ANGOULEME
The date of the Paris Exposition is not a clue to the age of the instrument. I've seen pianos that were marked on the iron plate “World's Fair 1910” but were most likely made in the years following the event. I'm not sure “MEMBRE” is right as the stamp is not clear. “LP” stands for Low Pitch, which equals A440.
This picture shows the forked E-flat/B-flat mechanism. Normally clarinets don't have a ring for the left ring finger. Holding finger 1 and 3 down is all it takes to play those notes, and boy, does that come in handy!
Note that the left pinkie keys are pin in hole, like Buffet clarinets, but the two keys are combined on one post.
That intonation is about as good as clarinets in this world get. I used a real reed instead of a Legere for this test. On my Legere, I was -4 to -10 flat on the A and B-flat throat tones. I would want a shorter barrel with this horn.
I suspect that Couesnon also made the wooden Pedler unibody instrument I reviewed earlier.
David Speigelthal at woodwind.org:
Couesnon was a decent French manufacturer, although they're mainly known for brass instruments. A. Fontaine, I believe (Mark, can you help us out?) mainly sold "stencil" instruments, that is, made by others but with the Fontaine name on them. I just restored a Couesnon "Monopole" clarinet (late 40's-early 50's vintage, I think), and it's a very nice clarinet. I can't tell from your post whether your instrument is a "hybrid" of Couesnon and Fontaine-labeled parts, or if it's a Fontaine stencil made by Couesnon. Sorry I can't help more than that.
Another interesting Link.
The bore is .580in or 14.75mm.
Yet another link.