Just saw this thread and I have a Stephen grant sidelever 20ga that is proofed as a 16ga. It was owned by Cyril adams. Do you have pics of your gun? Cyril owned Atkin grant lang and chamber sleeved many 16bores to 20 which made them more desirable here in Texas. I own 7-8 of them and all but the grant(which is Damascus barrel)were reproofed for 20 in the