Miyata CO2 cartridges
From Retrobike Wiki

Koga TrailRunner with the cartridges fitted.
Miyata made a specific CO2 cartridge assembly for their bikes. The assembly consisted of a holder, one or two adapter pieces for the right valve and two CO2 cartridges. The frames came with a m5 fitting under the downtube (double diamond frames) or under the drive side seatstay (e-stay).