My guess is that if it doesn't say Francotte anywhere on the gun, and especially since the proprietary Francotte proof (AF under a crown) isn't shown on the barrel flats, it's not a Francotte. Expect what you have is a Belgian "guild gun"--doesn't mean poor quality, but it does mean no name and therefore not worth as much. And because of the 20-70 in the sideways horseshoe or capital C, the gun (or at least the barrels) is definitely post-1924. A nice Belgian 20 with factory 2 3/4" chambers is still a gun that will attract interest, but certainly less than if it were a Francotte.
In your description, you mention pitting around the muzzles. Do you mean on the outside of the barrels or the inside? If on the outside and you post photos along with a for sale ad, you might want to try and get a decent closeup of the pitting so potential buyers will have a better idea.
