I looked through a few supplements but didn't see it. I don't have a complete AAR open car loading rule book.

These links might help:
Discussion and color comments:
http://prrthsdiscussionweb30239.yuku.com/topic/2596/Ship-s-propeller-flat-car-load#.VBM9SEj0Tzc

Photo: http://members.trainweb.com/bedt/bedt/bnyoversize1942.jpg

Photo source and description:
http://members.trainweb.com/bedt/BEDT.html#BEDT
search for "propeller" to find it on the page.