If you care to provide me with your shotgun's model (not serial) number I can check it against some cross-reference lists and perhaps definitively tell you who was the manufacturer.
"Western Field" was a house brand for Monkey Wards. From the time the Mossberg 500 was introduced, it was also sold under several "house brand" names. Wards may have sold High Standard pump guns in the pre-Mossy 500 days.
My Mossberg 500 is a "New Haven 600"! I think New Haven was a house brand of the old Western Auto stores. Took it in partial trade on a car I sold back in 1978, I think. Yes, it's only a plain Jane Mossberg house brand shotgun, but the car was only a high-mileage Ford Maverick- with working brakes on only one wheel. I'd bet that car was melted down for scrap many years ago, but the shotgun still does just fine!