Muffin,
You were right - the dwell plug of the timing light must be connected to the CB coil post otherwise no dwell signal is measured.
By then the dwell reads 22 but since the car is wired in positive earth, and the engine being a 6 cyl, actual dwell is therefore 60-22 = 38°. I replaced the points recently and adjusted them according to the manual, for what it's worth.
However, the 500 rpm error between actual engine speed and the rev counter seems more or less constant through the rev range. More than happy to live with it though, now that I know I do not really need the rev counter to monitor actual engine speed.
Also, for engine speed the timing light does not care whether its dwell plug is connected or not. It seems to be just calculated from the signal of the inductive clamp, as the number of cylinders is configurable when switching on the timing light (if the engine is 2 or 4 strokes can also be configured, for what it's worth).