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.
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Made by Mossberg for Montgomery Ward stores late 1950's to early 1960's
I have one here. It is shootable. I have trouble injecting a shell in the chamber once in a while...
Havlin Sales. They are available on their website. They carry many parts for older Mossberg rifles and shotguns. About $5.00 for the plastic peg, plus shipping.