It's E over LG for Belgian, but you shouldn't find it on the Helice gun, since it is French. I do not recall seeing any examples made after WW-2, and all the Helice shotguns I've seen before were proofed at the St. Etienne proofhouse.
Not many of this manufacturer's pieces are seen in the States, and pricing is extremely variable and will depend on amount of embellishment and overall condition. They are not 'Best-Quality' French shotguns, but they are still very nice pieces.
Values on boxlock Helice guns run $500-$1500 depending on gauge, condition, etc. Sidelocks can run considerably more. Finding a buyer at the upper end can be tough - French shotguns are a much harder sell than English or even Spanish in the same price range.