Polite Suggestion / Guess: A "Philatelic" cover, intricacies not in my knowledge.
Cover from a known stamp dealer and expertiser. (Finland and Germany)
Germany (1945 Flensburg local) Droese, Bernhard ('B. Droese') Plön/Holstein 1950, Düsseldorf 1961, Bad Salzuflen 1968, BPP 1958–
https://www.filatelia.fi/experts/areasf.html