There is no known record of the serial numbers of the old Couesnons (
pronounced "que-non," in case you don't know). Their records were destroyed by fire in 1969, and the horns have switched hands so much, no one really knows anything about the Couesnon dating. In fact, all the writtings and engravings on the bell, and the numbers really don't have a pattern (
wonder who was the dumbass who thought that up? "Hey, lets just confuse the hell out of our descendants by putting numbers in strange places! That'll show those flying-car riding bastards!" 
).
The older (
1890-1930, I'm guessing) horns (
and by that I mean trumpets, not knowledgeable on the flugels, percussion and woodwinds) had the date engraved on the bell, but I dunno when they stopped that.
If you want to know anything else about the history of Couesnon, an obscure piece of history, in my opinion, go to Mr. Donaldson's "Trumpet Gearhead," at this URL.
http://www.dallasmusic.org/gearhead/Couesnon%3F.html%20
Thats where I got the majority of the information I have been spouting.
(
Eh, a little late to post, maybe he's still around)
Van