Before sealing the meter by the engineer I connected a wire to the SO output (pulse).
The meter is telling that it has 1000 pulses a kWh.
So I connected the rfxpulse counter to the SO bus and directly started counting pulses

after a while I discoverd that the pulses where counted way to slow.
tuning the rfxpulse did not help...
after a while I found this foreign document telling me that on the SO bus there is 500 pulses every kWh

Working fine and very precise now!