A grade is the lowest with no engraving
B grade has a small amount of engraving (a little more than a PH Parker)
C grade (a little less engraving than a GH Parker.
D grade like a BH Parker
E grade like an AH Parker.
F grade a little less than a B grade Remington, this is a trap gun with steel barrels and generally a higher comb.

Here are some pics of different grades.

The bottom of a couple A grade guns



A couple B grades



A couple of C grades



A very nice E grade