The A Z plate is almost certainly auf (open) or zu (closed), so probably is to do with opening it, but I can't see how from the photos.
The best place to ask is the Kodak Retina and Retinette Lovers Facebook page - the Retinas came out of Nagel patents, so they welcome Nagel cameras, and they have access to a Nagel historian (not on social media himself) who will probably be able to tell you a lot about your new baby. Camera-wiki suggests it uses 120, btw.