This is the same gun as a Mossberg 500. You can try a Google search or go to the gun parts Co. online for a diagram of an exploded view. Mossberg should have one as well.
Your shotgun was made by Mossberg for the Montgomery Wards Company(Westernfield).The Mossberg model 500ab is the same shotgun as yours.
Mossberg, A= 12 Ga, B=Dept Store Model, R= deluxe. It's a Mossberg 500.
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.