In an early OECD study by Deppler and Ripley (1978), the authors distinguish between output per man hour, $OMH$, and what they call "Normal" output per man-hour, $NOMH$.
At no point in the paper do they explain this distinction, which also occurs for the labour cost variables.
In the text (p154) they explain that:
The cyclical component of labor costs is represented by the ratio of actual to "normal" output per man-hour.
This all seems very mysterious to me. Are they talking about a particular reference year value? Or is it a trend component?