The grade should be stamped above the gauge near the front of the left watertable. A Field Grade of that vintage should be stamped F or possibly S in that location. A Field Grade would have barrels roll-stamped "Smokeless Powder Steel" on top.

Posting pictures would surely help. With the 2 and 4 choke markings, and the slightly odd barrel length there is a good chance the barrels may have been cut from 28- or 30-inch.