BTW- I expect if there is a walking foot, it will attach like the buttonholer does - where you remove the whole top clamping unit and use the set screw to secure the attachment to the round pressure foot bar.
I've been looking, and so far the older walking feet have been Singer, and all side clamp. The concept of the walking foot has been around a long time (I have a veritcal feed Davis) but I don't think they were made as attachments on a regular basis until much later.
I know Singer came out with the Penguin foot pretty early on, but I don't see anything in between until they came out with the white one that is basically plastic.
Who knows - there may have been a walking foot for the top clamp machines, and it may be as rare as the penguin is - another fun thing to look for.