I think you're on the right track, but I don't think that's a washer. It appears to be part of the SS--or at least in direct contact with the bushing such that the SS can force it to turn. If so, then a flat washer between it and the nut may help prevent the up and down movement of the SS from forcing the nut to back off. And, good practice is that there should be at least two threads of the mounting stud past the end of the nut, BTW. Also: OP should use a torque wrench, not an impact wrench, to attach the nut. Proper torque is 50 ft-lbs. Finally, if it continues to come loose, drill the stud and use a cotter key, or drill the nut and use safety wire (but be careful of how you do that because there's a lot of up and down movement occurring there) ...

http://www.bobmartinco.com/pdf/safety_tying_wire.pdf