German map of Sicily

Good afternoon.
I am researching events in Sicily in the summer of 1943, and today I was looking into GAF claims against P-38s found at aces.safarikovi.org/victories/doc/victories-germany-ww2-claims-p-38.pdf. At that website, I see the locations and altitude of the claims clearly indicated, but I am not clear on the map references. They are typically 5 digits (i.e. 28114), but sometimes have 6 (8177/04).
I would be very interested to know what these map references are. To what map do they refer? And, is the map online or readily available?
Many thanks,
Robert