hints, including a book in Dutch of course titled Opkomst van de Nederlandse Radio-Industrie 1915- 1930 by Frans J.J. Driesens. It translates to "The Rise Of The Dutch Radio Industry..."
So with that book alone we can corroborate the postcard date range as feasible. The book was published to mark the 25th anniversary of the Dutch Society for the History of the Radio [NVHR]. I wish it was printed in a language I could read. But it's existence already tells me a lot.
here is loaded with images of radio expositions in Frankfurt, Rotterdam, Scheveningen, Berlin, and Amsterdam. Some are as early as 1925, others as recent as 1965. the brands displayed include brands I recognize like Rolls Royce and Phillips, but many others I do not such as Erres, Trolit, Zelander, Baltic, Dominir, Firato etc. Incidentally the banner in the back that reads "mubelstijl" means Furniture-style. How very 20s.