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Thanks for the photos. Your shotguns has the "US" and S prefix serial numbers (although the S numbers are mismatched) like the Vietnam era US contract M37 riot guns. But it also has sling swivels...

Your description sounds like it is one of the early 60s Model 37 riot guns made specifically for the Viet Nam Govt'. They are not "US" marked, and many were founds with S prefix serial number added...