A very good deal on a fairly rare phone. Not many 556 models have shown up, or at least not to me. I think they were made for a very short time and then both the 501 and 556 tube phones were discontinued and a different scheme adopted for party lines. I'm not positive, but the 556 models may have been made for only two or three years. That would explain why they're not often seen.
You are correct about it being too early for light beige. That would have been late 1957 and after. Good detective work!
Please post some pictures of the phone after the clean-up. I bet it will look great!