I THINK the 520 had a "humpback" receiver. This is a link to the cross reference page at numrich- you will need the Westernfield model number- but scroll down- there were a lot of guns made by Mossberg, Savage, marlin, etc. for the Westernfield name. I have a couple of nice High Standard Flite Kings with the Sears JC Higgins brand on them. http://www.e-gunparts.com/crossref.asp