G75X...Z...I...K...F...D... No decimal point in the "D". Two block G75 uses a decimal point with the R-value.
Hardinge uses M21 to open collet, but that doesn't mean that Okuma does. Call Okuma if you don't have a manual for your machine. Or maybe someone with an older Okuma will respond. Did you ask about it on the Okuma Forum?