The Banc D'Epreuves de Liege provisional proof on the thumb push guns is the only I've seen where it would be visible.
All the other tube maker's (not proof) marks, on every U.S. maker's gun, are on the bottom of the barrels just forward of the flats.

'JDH' is an unknown maker's mark on Lefevers

1892 G grade



The others are 'ADH' - Arthur-Delvaux-Heuse & George Laloux (posted on p.2).
Lots of examples
http://www.picturetrail.com/sfx/album/view/18490292