Hi,
Do you really need to know the curve for that specific group?
I see this curve as a generic one, just to show how the CTR behaves over LED currents and over each unity. Saying that your opto belong to "K" group means that its curve will be more likely "Sample A" curve. For other groups in which CTR spreads, the curve can be any. Each unit will have its own curve.
Once I worked with a common EL817 optocoupler and there was huge variations in CTR. It was an analog signal read by a MCU throught an optocoupler. In order to compensate CTR variance, I made an auto-adjust routine. In production, a known input is applied to the circuit, and the firmware looks for a gain that causes the desired reading for that input. Then this gain is stored and used during normal operation.