Hi Re-Pete and AlinCal:
I don't remember where I read about the function of the air deflector--possibly in the owner or parts manuals for our 1973.
According to the parts manual for our 2009 Flying Cloud, page I-24, this item is part number 104948 and is described as "Air deflector, between roof A/C and refrigerator".
Unfortunately no schematic is given of the roof component lay out in the 2009 parts manual. The refer vent and cover are on a different page, IV-44.
One difference between the 1973 manuals and the 2009 manuals was extensive explanation of some features/functions and maintenance procedures in the 1973's. The 1973 parts manual was actually titled "Service Manual" even tho most of its contents were parts lists and schematics like today's Parts Manual.
"To err is human, to air is devine."