Well, you learn something new every day. I had never heard of this instrument before. A bit of research uncovered the following.
"The Opheclide served as a tenor/bass pitched transition instrument between the keyless and keyed Serpents of the 1700s and the valved lower brass instruments of the mid 1800s. The idea came from the keyed bugles first invented in 1817 in the UK. The first Opheclide was patented in 1821 in France and a version was still listed in the French Cousenon catalog of 1910. Essentially it is a tapered tube about 8 feet in length and pitched in C (or sometimes in Bb) with a bell about the size and shape of a alto or baritone horn. They usually have 9 keys placed so the player can open and close a single or combination of large holes in the tubing - using two hands- much like the saxophones of today- but with the air column vibrations coming from the players lips using a euphonium size mouthpiece. Higher (alto) and lower (contrabasse) versions were also made."
http://www.rugs-n-relics.com/brass-p...opheclide.html
Apparently Berlioz' s 'Symphonie Fantastique' has a part for one of these.
I see what you mean about the date inscription. The comparison makes the number clearly a '3' and not a '5'.