I came back for a second crack at making FAB-100 bomb fins, their only parts to be visible, inserted in their vertical drop-box in the bombay:
All together, a huge loss of time, and failed attempt. However these are the thinnest castings I've ever done - and now I know the limit!
Meantime, I found yet another relevant website, including specific references to the Chinese SB-3s. I am also continuing to search/translate (from "Simple Chinese"), the fairly extensive text at the Soviet Volunteer Air Force web pages:
Great stuff - and you can definitely see the SB's were engaged in maritime/anti-shipping operations around China!