For the non-Dutchies here on the forum: the article lets Vincent Meylan stating in his book that King Willem III of the Netherlands, Grand Duke of Luxembourg was a great admirer of the designs by Oscar Masin.
M Masin was a designer at the famous jouaillerie Mellerio dits Meller in Paris. Many pieces in the royal collection were purchased at Mellerio but apparently not the diadem worn by Queen Máxima at Investiture Day.
The article writes about Vincent Meylan's discovery or claim that it probably is a design by Oscar Masin indeed but executed by another firm.
The monture worn by Princess Mabel at her wedding day was from Mellerio, it originally had a mix of white and brown (???) diamonds but later was all white diamonds.