Bob Schaller told me about it - in his 'More BS...' he simply refers to it as 'a Volvo six blade fan (which is no longer available except in jink yards)'. This was 32 years ago so that may give you some idea of the era of Volvo involved.
There are other options that are possibly easier to find. The fan from Macy's is one, a funny-looking BMW fan is another. I used 6-blade STC Tropical fans.