I found a service manual for the SL1200mk7. It seems one should be able to measure speed related signals directly from pins 1-3 on the main PCB. Speed is apparently measured using some kind of Hall sensor. The manual does not really give any details on what the different pulse signals are.
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Likely the hall sensors are for commutation. There's like a separate speed sensor, or there are in their other designs (GR, G(AE))