Your shotgun was made by Mossberg for the Montgomery Wards Company(Westernfield).The Mossberg model 500ab is the same shotgun as yours.
The 12 gauge Western Field Model M550ABR was produced in the early 1970s. This shotgun was indeed made for Montgomery Ward, a retail company that sold firearms under the Western Field brand. It was manufactured by Mossberg, which provided several models for Montgomery Ward's catalog.
Mossberg, A= 12 Ga, B=Dept Store Model, R= deluxe. It's a Mossberg 500.
The Western Field 12 gauge shotgun model M550ABR was manufactured by the Mossberg company. This model was produced in the early 1970s, specifically around 1971. The shotgun was sold under the Western Field brand, which was a label used by Montgomery Ward for firearms. The serial number G780900 indicates it is part of that production era.
It's a Mossberg 500. This model has been made since 1962 in several configurations. The company may be able to give you the date of manufacture from the serial number, but their lists don't seem to be available to the public. With the Mossberg name, in NRA Very Good condition, they retail between $200 and $375, so re-branded for Montgomery Wards, they should go between $140 and $300.