Piero,
I measure 6.010" between the stanchions and the stanchions measure 1.354" so the distance center to center should be 7.364". My calipers do not reach fully to the center line of the stanchions, so the measurement between the stanchions may be slightly less, but not much. Metric 152.65 between and 34.40 stanchion, so 187.05mm center to center.
Measurements made just below the lower tree yoke.
Dave