Hi Krisztián,
Huey & Dewey indeed have a Belgian fineness mark A(rgent) 835 and Belgian responsibility/sponsor mark, a steam locomotive, 5 wheels (3 big, 2 small) on a piece of rail in a barrel for Henriette Renaerts (widow of Boland Georges Emman), registered in Bruxelles, 21-2-1957-ca 1962. The duck heads are filled, 4g in a triangle or 4 gram of silver used. HB under crown could be a maker's or retailer's mark.
Peter.
Source; Walter van Dievoet Algemeen repertorium van de edelsmeden en van de merken van edelsmeedwerk in België. Vol. III: 1942-1997. / Repertoire general des orfevres et des marques d’orfevrie en Belgique # 0832 page 88