As far as I know, there isn't any in deep history of the red cross italian aircraft and their units/organization. You can find some details in the histories of some aircraft, especially CZ.506.
The aircraft at this link
https://www.ebay.de/itm/373902291633...UAAOSwgWJh7-Mh
is a S.81, not a S.73
ciao
Gianandrea