Bob Beach was the Archivist for Griffin & Howe, was an invaluable Francotte resource, but unfortunately for the vintage double community has retired
https://www.shotgunworld.com/threads/abercrombie-fitch-von-lengerke-detmold-and-francotte.393236/

He discussed the Jubilee guns, which were not cataloged until 1941, here
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=361368

There are several guns listed as Jubilee models right now on Guns International, at least 2 of which do not include the SN or pic of the barrel flats
https://www.gunsinternational.com/s...rancotte&start_row=1&the_order=6
A boxlock
https://www.gunsinternational.com/g...2-gauge-s7950-quot-.cfm?gun_id=102347781

Per the link that Mike provided, your SN fits the "Medium Grade" models for 1905 or 1906.
The No. 20 was introduced in 1910.