For the one of us still interested in this, looks like
Henri Pieper and
Louis Muller were the major Belgian makers of Folsom imported guns. Muller registered twelve trademarks 1889 to 1894 including: Acme Arms Co., Club, Climax, Star, Crescent and Crescent Arms Co., Rival, Standard, Meteor, and I.X.L.
Here's a W. Richard by Muller (at least he added 'Belgium')
http://www.littlegun.be/arme%20belge/artisans%20identifies%20m%20o/a%20muller%20gb.htmWilmot by Peiper
http://www.littlegun.be/arme%20belge/artisans%20identifies%20p/a%20pieper%20henri%20gb.htmHere's a Lefaucheaux Action W. Richards, but I don't find 'DD' on PeteM's list of marks

DD in an oval lying was used by
Jean Delcour-Dupont de Fraipont-Nessonvaux
Stanley Arms Co. was made by
Francois Dumoulin http://www.littlegun.be/arme%20belge/art...%20cie%20gb.htmLater T. Barkers may have been made by
Theate who's mark was 'TF' in a rectangle
http://www.littlegun.be/arme%20belge/artisans%20identifies%20t%20w/a%20theate%20freres%20gb.htm