Huh?? What? I've never heard of a Savage built John M. Browning designed pump. The humpbacked J. Stevens made John M. Browning designed pump is the No. 5xx --









And, I've never seen them catalogued in anything but 12-gauge. Their 1927 replacement the streamlined No. 620 was introduced in 12-gauge with 16-gauge coming the next year and 20-gauge the year after.

The Savage Model 220 was a hammerless single barrel first introduced in 1937, along with the very short lived Model 320 side-by-side, and Models 420 and 430 over/unders.