The info pages on that site say:
"in schiefachsiger Azimutalprojektion"
which google translates to:
"in a slanted azimuthal projection"
Which I suspect means an "oblique" azimuthal projection. Not sure how you'd get the rest of the parameters but if you need to georeference the maps there should be no problem using the grid intersections as reference points in a non-affine transformation.
A bit of image processing to enhance the grid lines shows there are no straight lines of longitude (meridians, N-S lines) in the map area. The meridians are getting straighter from left to right so I suspect the meridian that defines the projection is just off the map, and the oblique angle is the angle that would be at. But tricky.