Hi,
Photos of the marks would help, as the marks you describe are not consistent with either Belgian, Dutch or French proof marks. One of the markings is the customs property mark, see below.
A foreign gun, proofed in Belgium would have the crowned cartouche with a styled L at least once on it. A gun made in Belgium and proofed there would have the ELG/cartouche as described earlier.
Does your gun look somewhat like this?
These pistols were introduced for the Belgian customs offices in 1901.
By royal decrete of the 3rd of juli 1901 it was said:
-Customs agents, lower brigadiers and brigadiers were equipped with a carabine of the Terssen design, including a bajonet and a Nagant revolver with a stamped customs marking.
-Lower leftenants and higher officers wore an infantry officers sable.
-Customs agents, lower brigadiers and brigadiers were issued a leather belt with silver buckle, sable holder, carrying belt for the carabine and a leather cartridge pouch.
-While wearing ceremonial dress the brigadiers were allowed to carry the sable of the adjunct of infantry with matching belt and garnitures in white metal.
This reference in Dutch was found on the website of the 'The Belgian Army 1830 - 1914',
http://users.pandora.be/ABL1914/index.htm