1935 Flat side. 1 1/2 gallon center fill
1936-1940 Round side. 1 1/2 gallon center fill
1941-1944 Round side various capacities from 1 1/2 - 2 1/2 gallon. Center fill
1945-1951 ish. 2 1/2 gallon right side fill screw in cap
1952-on 2 1/2 gallon, both single and twin cap tank, most think that the single cap were from the 3 wheel trucks, but have seen on other machines. The single caps can also be foundin both left and right fill, so who knows. These caps are 1/4 turn caps.
This info comes mostly from Gary's book.
Rick