It would be an enormous help if Bob Beach would look in.
His Francotte tutorial is here
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=229207I don't know when Auguste Francotte & Cie began marketing Francotte Arms guns, but here is a thread with a 1961 gun
https://www.shotgunworld.com/bbs/viewtopic.php?t=292163The
lettre annale is a lower case theta for 1953.
Here is another Francotte Arms
https://www.liveauctioneers.com/item/60893502_francotte-liege-belgium-double-barrel-shotgunP1K379.5 is the barrel weight at the time of proof = 3.041#. If the barrels now weight > 3% less (from honing) the gun is considered to be out of proof.
The original bore was 16.9 mm = .665"
R barrel constriction is 16.7 = .657" = .008" choke = Improved Cylinder
L barrel constriction is 16.4 = .646" = .019" = Modified
It has 65mm = 2 1/2" chambers
Siemens Martin steel is a quality product