And Royal Magazin says that the tiara was purchased in 1921 by Prince Karl Egon V. zu Fürstenberg for his bride Countess Franziska Ida "Mena" von Nostitz-Rieneck.
I could understand the discrepancy in the years 1912 and 1921 but Karl Egon and Mena didn't have any children and therefore could not have had any grandchildren. Is there something I'm missing?