I couldn't easily find the thread size on-line, but here are some suggestions:
- It is likely a metric size...maybe identical to a larger brake line fitting - like M11x1.5mm, M12x1mm, M13 x1.5mm
- It is the same size thread as the CDV and CDV-delete valves...UUC, Pelican make and sell them...ask them ?
- Take the clutch line (male) and a BMW slave cylinder (female) to a hydraulic or speed shop and see if you can match it up...
- Maybe send FTE or ATE (OEM manufacturer?) an email request?