Indeed, "canon de surete" is singular. But I believe you'll find it marked on only one barrel of a double--which has two. Same for all the "canon" markings found on the guns JC shows. Darne describes its guns as "fusil juxtapose a canon fixe"--yet both barrels are "fixe", not just one. Charlin uses the same "canon de" (singular) in its ads that Darne uses when talking about barrel length--and there are two of them, but they tend to be the same length on the same gun.
On the other hand, I find Darne material that reads "canons fixes". A Verney-Carron ad I have reads "Canons d'acier diamant", but the Manufrance Ideal ad reads "Canon acier special HERCULE". In other words, you'll find it both ways, used pretty much interchangeably, when referring to a 2-barreled gun. Which goes back to my point that the fact this particular gun says "canon Faure Le Page" doesn't really tell you anything about its authenticity, or lack thereof.
