I will double check but I'm pretty sure there is no 3 1/2" 20-bore Proof.

The best you will do is 3", thought why the heck anyone does this kind of amateur 'gunsmithing' on good kit is beyond me.

You may be able to have the barrels chamber sleeved to 2 3/4" and proofed accordingly. Depends how badly the half a**ed job was done and what degree of concentricity the dullard has stumbled upon.

www.vintageguns.co.uk