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Well, while Kai Winkler is an experienced knowledgeable dealer, he's hardly an expert on Freikorps.
Actually, I know what this is. It's the collar badge of the Zeitfreiwillige der Reichswehr Brigade 4. The Reichswehr-Brigade 4 Magdeburg of the Vorläufige Reichswehr was formed in June 1919 from the Freikorps unit Freiwilligen Landesschützenkorps.
Here is the collar badge of Freiwilligen Landesschützenkorps/Reichswehr Brigade 4 shown next to the collar badge of Zeitfreiwillige der Reichswehr Brigade 4.
Also, here is letterhead with the logo of Zeitfreiwillige der Reichswehr Brigade 4.
Looking at the stickpin again, there's a slight possibility that this is the civil stickpin worn by former members of Zeitfreiwilliger der Reichswehr Brigade 4. It likely can be ID'd definitively in the Katalog der Abzeichen deutscher Organisationen 1871-1945 (Catalogue of German Membership Stickpins and Badges) by André Hüsken.