It is the serial number for the tube set, not for the gun.
Bernard's numbering system was sequential for all barrel sets made.
Didier Drevet was probably no different.
Towards the end of Bernard's company, he replaced dates with code letters (a la Belgian proof mark date code), probably so that customers would not complain that the tubes were much older than the gun. It is common to have one year difference between the tubes and the gun manufacturing dates.
I have seen on this board an example where the gun was much younger than the tube-set (by over 10 years IIRC).
It was common for the barrel maker to make a barrel assembly and sell it with ribs and all to the gunmaker. This is probably OK for shotguns, but not OK for a double rifle. My Rieger DR with Bernard barrels was not regulated whatsoever. JJ put that to rest 115 years after it was first sold .
Best regards,
WC-
