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it is a shot gun not a rifle
This is the same rifle as the Mossberg 810.
Probably one available from Numrich Arms (e_gunparts.com), but you should probably let your local gunsmith replace it, so you might as well order it from him.
Tesco, Brighton
Numrich gun parts
The Western Field mod SD57 was manufactued by Stevens as their model 87. Limited parts are available from Gunpartscorp.com. Look around for other sources.
Your rifle which was a Mossberg model 320 which was made for Montgomery Wards(Western Field) was made from 1960-1980.Any firearm made prior to 1969 did not require a serial number by law.That may be the reason you cannot find one.
You can find a firing pin for the ranger at Numerick Gun Parts Corporation.
From what I can find out it was made by Mossberg and is the same as the Mossberg model 20 and 21.
Made by Mossberg. We have magazines. gunshop@nctv.com
From Henry
I would start looking at numrich gun parts corp.