That should mean the horn was built in 2015, I believe.
Going forward, the serial number on the receiver has no relationship to year. I received this message from besson in November 2022, referring to a serial number that has no date code in it:
“I checked with the factory and it seems as though they had made the change to our Besson serial numbers sometime earlier this year. This was done to conform with the rest of the brass instruments made in our German facility (B&S trumpets and tubas, Antoine Courtois trombones, Hans Hoyer horns, Meinl Weston Tubas, and Scherzer trumpets).
This change happened alongside discontinuing the stamp on the valve casing. “
Mike Taylor
Illinois Brass Band
Fox Valley Brass Band