Janey,
ISMACS didn't even list Portman! I did find a reference on some machine collecting blog to National's late 1930s aluminum lost cause (the featherweight consuming its intended market, after WWII)...From the pictures, it looks as if the tiny free arm is curved and pointed.
I just figured out what it reminds me of -- maybe somebody with an Airstream trailer would want a sewing machine that looks like an accessory.