It was proofed in 1927.
I see marks from 2 makers.
Jean Falla, 35 rue Bonne Nouvelle, Liege. He was around until 1935 at which time his son took over the business.
http://www.littlegun.be/arme%20belge/artisans%20identifies%20e%20f/a%20falla%20gb.htmAndré-Joseph-Alexandre Thonon, 65 Rue St Laurent Liège
Trade mark consisting of a four-leaf clover over the initials EL.
Andre was part of the very large and famous Thonon family. At least of member of the family owned a patent on a damascus pattern that was known as Thonon Damascus. The family was around from over 100 years.
Namur is about 50 miles SW of Liege. No idea if Thonon had a shop there.
As for value... Teddy Roosevelt hunted Africa with the .405. A double in this condition, I would guess is worth at least $15,000. But you need to check around to be sure.
Pete