Nicely done, James! Yes koppa just refers to a size so you could call a small Arkansas stone an Arkansas Koppa
It is the type of stone that is of interest - in this case an Ozuku Asagi.
The stamp you see in the picture above says Tanaka Toishi Kyogyosho of Kyoto. Stones with these stamps are imported exclusively by me and sold through Mark / CKTG They serve as a mark of authenticity in that Tanaka certifies the stamps on these stones to be from the mine specified, giving you an assurance of quality. As he owns the stamps. I cannot go out and stamp stones with this mark - nor can anyone else.