Around 1957/1958 the Comité voor Belgisch-Nederlands-Luxemburgse Samenwerking (Committee for Belgian-Dutch-Luxembourgian Cooperation) commissioned a Dutch flag manufacturer to design a flag, and the latter copyrighted this design. Because of this the flag did not become popular, and I have never seen this flag used. The flag is a composition of the three country's flags and coats of arms; it is horizontally divided red-white)blue, with on the white stripe a black panel. Overall is placed a yellow lion rampant, with a red tongue and white claws.
There is another artistic rendition of this flag, shown on the flag plates of the Winkler Prins Encyclopaedia (Elsevier), 6th Edition, 1954 (vol. 18, VEN-ZIJZ, Vlaggen plate, p.180-181). There, the lion has a less flamboyant design and is comparable to the one on the South Holland provincial flag.