Check this article out.
http://www.fourwheeler.com/features/...kward-glances/
I know it is more geared toward the K series, but it is the best one I have found regarding good info on RPO codes for 1972 trucks. Again I don’t know source of info so take it for what it is, but I am going with it being solid info.
According to the article, RPO code ZW4 is a Muncie SM330 3-speed. That info is about half way down the page.