@Markw posted:The best article I ever read on this subject was CTT issue February 1990 that has a ten page summary of all the shell, frame, motor variations over the entire postwar run of the SF F3.
I believe that article was written by the founder of Mountain Lakes Trains in NJ, at least I think that is the correct name. He was quite the authority on the PW F Units. I think he had even done a video on them at one time but that was 30 plus years ago.