factory (if memory serves) I think is a wingbolt. Might be hard to find but contact dealer or Canadian supplier. You could make a new one, if you can find a long enough shank - wingbolt or cap or suitable looking flat screw to cut off the imperial ends, (and find a die) to rethread the correct metric diameter and pitch, re-blue then just screw in. You might get lucky and find a metric screw that fits in all respects that is also in the gray.
EDIT: since you are not averse to boring out the swivel threads, you could do that then just take the swivel, stud and correct diameter pin to someone who breaks and spins chainsaw loops: he could snugly spin caps on each ends of that pin to never come off again, then just touch up with cold blue. I favour measuring and finding the correct screw even if slight modifications to it are necessary: they are nice looking sling swivels unadulterated. Or just have a smithy make a reasonable copy of what was originally there. Another option is remove the stud from the stock and buy an Uncle Mike's machine screw and nut... should fit in the hole.