I have a viewcard with the FPO 159 which Proud does not list for 1946 but as location North Africa for late 1942 until the Germans surrendered in Tunesia. However, the writer states that during the weekend they take a similar road as depicted in the viewcard to ski in Mendola and that the next day he would go to Venice. That makes the FPO 159 location to be most likely somewhere in Trentino in February 1946.
Marc
All I can add is that a listing I have shows 159:
28 June 1946 - 27 L of C
19 Apr 1947 - HPC RE
27 L of C refers to 27 L of C Postal Unit which The National Archives war diaries shows was in Italy (the war diaries only go to June 1946, thereafter they were quarterly historical reports.
This appears to support your theory, but a view of the war diary is necessary to ascertain the location of APOs ran by 27 L of C Postal Unit.
Mike