Hello Hallibag,
Welcome to the forum, and thanks for posting this excellent buckle. That's great to have a provenance with it. One of the Prussian equivalents of this buckle in my collection has a provenance of it being brought home by a local WW1 U.S. veteran.
I'll post some photos of what the spool of wire device looks like when attached to one of these buckles.
I apologize that the photos are not the best quality, as they're photos that I shot from the excellent photos of this device shown in the book titled: "Imperial German Field Uniforms and Equipment 1907-1918" Volume 1, by Johan Somers. The spool of wire device, which is called an "Aufspuler" belonged to the late Chip Minx collection.
I'm sending you a private message here as well.
Best Wishes,
Alan
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