The Mithraeum I in Stockstadt contained images of Mithras but also of Mercury, Hercules, Diana and Epona, among others.
Stockstadt mithraeum is not located at Saalsburg Kastell. Stockstadt is at least 30 Kilometers from Saalburg to the southeast. Stockstadt had its own fort (kastell) on the Limes. I wrote an article on Stockstadt fot Our Common Sun about ten years ago. I have been to Stockstadt and there is an excellent museum there.
Many thanks for the information, John. The page has been updated.