Dumoulin has stopped. I believe the last generation to still build rifles, passed away some years ago, and his wife is selling off the estate. As far as I can tell, there is no website, or easy way to contact her anymore.
Currently in the category of still active Belgian fine arms builders that I know off are:
Marcel Thys
Alexandre Hanssen
Paul Pletsers
The first one, Thys, is world renowned. The latter two are younger and still need to make their name. But for for far less than a Rigby big game, you can have a fully custom rifle engraved. Custom, of the highest quality, but without the Rigby price tag (nor resale value either of course)
There might be some other makers still out there, but difficult to tell for me.
A Paul Pletsers 416 rigby, asking 22500€
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A Hanssen 375 flanged double rifle still in the white:
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