In American it would just be Corp, so that suggests they are following the British system. But the short form for a Limited Company in British English is just Ltd, and I don’t think I’d even seen Co. Ltd. before I came to Japan. Some total guesses:
It is set in Japanese law as the official English translation of yugen kaisha (有限会社- lit. ownership limited company), like the also odd Diet and Prefecture.
It used to be that in British English, and has changed in the UK but not in Japan.
Both me and my Japanese students are dying to hear more theories on this one.