Can someone please explain how to differentiate the various types of single stage airpumps? For instance, I have seen circa 1930s pics of C-21 #361 with air pumps which have what look like cooling fins on the bottom half vs. pics taken in the 40s with no fins. (I have an Otto Perry pic of the #361 which shows both sizes/types being on the engine at the same time.) I assume this is a size difference? What sizes were the two?