Technically, SS-Pol. Stu. Btl. 13 would be SS-Polizei-Sturm-Bataillon 13- not Sturmgeschütz because the Sturmgeschütze units were organized into Abteilungen, not Bataillone. But this is academic because Lorenzo is right that there was no SS-Polizei-Sturm-Bataillon 13, and thus he's also right that the disc is likely a fake.
Let him who is without sin cast the first stone.
Pluralitas non est ponenda sine neccesitate- "Entities should not be multiplied unnecessarily'' William of Ockham