[Python] First occurrence of element in a dictionary of sets

Given is a dictionary of sets that are contained in a list. What would be a pythonic way to get the key of the first appearance of an element in a set. Do this for all elements!

Example: Given is the dictionary d and element 510. The answer would be that for key 343 this element appears first in d. This shall be repeated for all values in the sets of d.

d={
100: [{406, 491, 440, 448},{590, 633, 560, 564, 651, 681}],
343: [{366, 412, 440},{448, 491, 510}, {557, 560, 544, 564, 633, 681}],
423: [{440, 448, 491, 510, 544, 557, 564, 681}],
433: [{412, 440, 448, 491},{536, 544, 510, 557, 560, 564}],
493: [{1286, 1285, 1357}]
}

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