The KOO-Tiyoko is a leaf shutter certainly made by Chiyoda Kōgaku Seikō (predecessor of Minolta) for military use. "Tiyoko" is an alternative writing for "Chiyoko", abbreviation of Chiyoda Kōgaku. The meaning of "KOO" is unknown.
The KOO-Tiyoko gives only 1/100, 1/200 and 1/400 speeds, and was probably made specifically for aerial cameras. It has a black front plate, with the name KOO–TIYOKO at the top and no provision for an aperture scale. One example is known with a Hexar Ser.II 75mm f/3.5, presumably mounted on an Army Type 99 Ultra Small Aerial Camera (GSK-99).[1]