I'd say you need the correct washer to match your cyl. head number.
I don't have my parts book handy,but they do list all 3 (i think 3)head casting numbers that fit those tractors. And it does list the matching washer that matches the different later year cyl. head drain back holes. All 3 of those cyl. heads interchangably fit from what like ser# 96,000, 1941 ? clear up until the last B made.