I believe the reason your gun is a medium grade is because it's a sidelock which was, and probably still is, considered the only action for best guns. An un-engraved sidelock was considered a higher grade than an engraved boxlock, within the Francotte hierarchy, not just an area for more engraving.
A shame about yours, looks like the drill holes are too far back to even have the gun sleeved.
For reference this #86423, a 20ga ordered through VL&D and delivered to A&F, who had just taken over VL&D, on Dec. 24, 1929. I have the complete documentation from A&F.
It's listed as a "Deluxe Ejector Grade" and again as a "Grade 140E.A.H. Ejector.The engraving is signed by Hippolyte Corombelle, a top Belgian engraver at the time. Looks to be perhaps a grade lower than the 12ga high grade you posted. Sale price in 1929 was $750.
