The rank crown shown here did not technically exist; even royal crowns looked different. The crown of a grand duke consists of a gem-set gold headband with five visible pearl temples and leaf tines, interrupted by smaller pearl tines (which are missing here). Behind the temples, half is filled with a purple cap.
I haven‘t seen another buckle with a crown with 5 temples, which misses the small bows with the pearl tines. Nevertheless this one still looks authentic.