Gerry:
Bob Beach will be able to elaborate, but the fact it doesn't match the description of the gun w/ that serial # in his Francotte records is a worry, as is a 1920 serial # w/ a 1931 date stamp, and finally the lack of 'crown over AF' makers mark.

example of AF makers mark here:
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=208093

regarding the importance of that mark:
http://www.cheaperthandirt.com/everygun/GunMfgDetail.aspx?id=377


Always looking for small bore Francotte SxS shotguns.