The FW penguin is for a short shank machine. However, what is unique about them over a generic short shank walking foot is that they are for the narrow feed-dogs that are rather unique to the FW. April 1930's Featherweight Shop carries a modern walking foot for the FW (not antique) for $20-25.
My experience with having the penguin came at the estate sale of a lady who worked for Singer teaching classes and doing demos. She had two of the penguins which I bought. One did not work, so I gave it to a friend who was able to fix it (the spring had come detached). She has been forever grateful! The other one I sold on Ebay. This was nearly 10 years ago.
My pleasure comes in finding and briefly owning this type of fun item, not in keeping them forever.