OK, you win. I couldn't find anything about a Skeet Grade in my Model 21 stash. However, those of us who have been buying Model 21 Skeet Guns for some time (for me, 44 years), we know a Skeet Grade from a Field Grade from across the room. Now we know enough to call them Skeet Guns. Even later Trap Guns are not Trap Grades. How about that. However, it is hard to tell the difference since both are stamped TRAP on the floorplate. I have never heard anyone refer to the DUCK as a Duck Grade since it is indistinguishable from a Field Grade except for the chambers.
