The parts book may be of some help, the 6039 model was very popular . . . 😀
This gear case was designed for the World War Two Storm Boat Motor and is virtually indestructible.The Elto pattern gear case designed in the early 1930s was used on pre war models. It has a 0.788" diameter prop as apposed to the 0.989" prop shaft on the Storm Boat Motor prop shaft.
When the supply of Storm Boat Motor gear cases ran out (They had built thousands under a government contract for storm Boat Motors in anticipation of invading Japan. The contract was canceled when Japan surrendered after being hit by two Atomic Bombs. The remaining Storm Boat Motors were converted to Big Fours and sold from 1946 through 1949.) A slimmer gear case was introduced that had noticeably less drag.