As an update on this particular pistol,.... I sold the pistol at a gun show and made a little money.
As for the exact size of a round ball fitting the bore of a smooth bore proper, there is some leeway, in fact shooters in the past often loaded their smooth bores without a patch. (unlike a "rifled" bore)
Instead, they simply dropped the ball down on the powder charge followed by a wad (of some sort) to keep the ball seated on the powder.
Of course, a shooter loading in that manner needed a ball that wasn't ridiculously smaller than the gun's bore (say greater than 0.020" under-sized), but it does work pretty well otherwise.