Ok George,

Your gun is a No. 520A with the flat-topped reciever. Doesn't have the second bump along the top like the No. 520 - 535s. It never appeared in any J. Stevens Arms Co. or Savage/Stevens/Fox catalogue that I've ever seen. I've seen them as military riot/trench guns marked 520-30 and as "trade brand" sporting guns for both Montgomery Ward and Sears, Roebuck & Co. The are in the 1951 Savage/Stevens/Fox parts list --