[Ron Ramirez]

So I have a 38-40K. Why K?

I have a little information about the various letters Philco used to designate their cabinet styles here.

How does it differ from the 39-39? The cabinet seems the same, but the 38-39 is cheaper. The radio must be different.

I think you mean 38-39. The 38-39 was designed to operate from a 6 volt storage battery. The 38-40 was designed to operate either from a 6 volt storage battery or conventional 110 volt AC current. So a 38-40 could have been purchased by a farmer who did not have electricity yet, but anticipated getting it soon. (Many rural areas of the country did not have electric service available in the late 1930s.)