Hi Steve,
Congratulations on this buckle purchase.
I'm not quite understanding what this Prussian buckle is from seeing your excellent photos, so I will ask some questions.
Is the buckle made of steel? Does it attract a magnet on the buckle box/frame? And on the applied disk?
I cannot determine if this buckle is plated, such as with a neusilber finish to it, or what the plated finish is made from after viewing the photos. It looks like the disk is ?pewter or gray in appearance.
I'm aware of the 2-types of Bavarian police buckles (rural and city), but I don't know what defines a Prussian police buckle since I have never seen a reference to it in any books that I have.
I do have a couple of Prussian buckles that are unusual and similar to this one of yours. One is neusilber construction of the buckle box/frame. The other buckle is plated with a finish made to look like it has a neusilber finish, though I don't think the plating material is neusilber. It has an appearance similar to your buckle. I have never learned what the use was for these two buckles I have.
I could dig out my two examples and take photos to post here for comparison.
Best Wishes,
Alan