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.
Numrich gun parts
If there are parts out there these people most likely have them. Numrich Gun Parts Corp. I have used them several times and there good people to deal with. http://www.gunpartscorp.com/
Vintage gun parts such as the needed magazine for a Model 46A Western Field 22LR can be found at major gun shows or through online specialty companies such as APEX and Numrich.
Numrich Gunparts.
it is a shot gun not a rifle
You can find parts for a .22 caliber Western Field rifle at various sources, including specialized gun shops, local firearms dealers, and online retailers like Brownells or Numrich Gun Parts. Additionally, websites like GunBroker or eBay may have listings for specific parts from private sellers. Checking with firearms forums or groups dedicated to vintage rifles can also connect you with enthusiasts who may have spare parts.
This is the same rifle as the Mossberg 810.
You may be able to find a magazine for a Western Field model M730 30-06 bolt action rifle by checking with gun shops, online gun parts retailers, or auction sites like eBay. It's important to ensure you are purchasing a magazine that is compatible with your specific rifle model to avoid any issues.
You can find parts for many westernfield shotguns at Numrich gun parts. WWW.e-gunparts.com
Tesco, Brighton
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.
I can't find a cross reference to the Western Field mod 804. If you can identify the manufacturer of that rifle (because Western Auto did not manufacture any guns - they bought existing models from a variety of manufacturers and had the Western Field name placed on them instead of the actual manufacturer) then parts can be procured from several sources. I suggest that you consult a gun values book to determine who made the rifle. Mossberg, Savage, Marlin, Noble, Stevens, and Colt all sold different guns to Western Auto. Possible sources of replacement parts are: Numrich gunpartscorp.com Jack First jackfirstgun.com Bob's Gun Shop gun-parts.com Order the parts by the actual manufacturer and correct part number. Some of these sites have exploded diagrams to assist identifying the parts.