The John M. Browning designed No. 520 was introduced by the J. Stevens Arms & Tool Co. in their 1911 General Catalogue No. 53. It was offered in a number of grades from the basic No. 520 to the 535 --
J. Stevens Arms & Tool Co. was bought out by New England Westinghouse for wartime production during WW-I and became the J. Stevens Arms Co. They were still offering the full line from No. 520 to 535 in February 1917. By 1919 the higher grades were gone and only the No. 520 and the No. 522 Trap Shooter grade were offered. Not long after the Great War, J. Stevens Arms Co. was bought by Savage Arms Corp. They continued to offer the No. 520 and No. 522. By their J. Stevens Arms Co. Catalogue No. 57 dated 1927, the double hump No. 520 had been repaced by the streamlined No. 620 and the old No. 520 was being marketted as a Riverside Arms Co. No. 520. Through this point in time these guns were only offered in 12-gauge. By 1928 only the No. 620 was offered but the offerings were expanded to include 16-gauge. Between 1928 and 1930 the No. 620 lost the suicide safety and got a crossbolt safety behind the trigger. By 1930, 20-gauge was added to the No. 620 offerings.
None of my J. Stevens Arms Co./Riverside Arms Co./Springfield Arms Co. paper from 1917 to 1941 shows the single hump pump gun. Neither does any of my post-War Savage/Stevens/Fox/Springfield catalogues. I have never seen any of these smoothed out single hump pump guns as sporting guns, only as riot guns marked 520-30. However, I just picked up a November 1951, Savage/Stevens/Fox parts catalogue and page 37 is for the Stevens Model 520A --