It is pretty strange that Astrid, a royalborn Princess in a reigning royal family did not wear a diadem. Both her own family as well the groom's family have nice jewels.
It is also strange that Donna Paola of the princes Ruffo di Calabria wore no diadem. We are speaking about the 1950's, still a time where grand gifts in jewels were common. Strange that she wore no diadem from the groom's family, nor was given jewels by the Belgian society, aristocracy or industry (diamond-rich Congo was still Belgian).
Anyway, good chance that Donna Elisabetta Rosboch von Wolkenstein will wear her mother-in-law's Savoia-Aosta diadem.