Cetrek 609/619 Autopilot - Rate stepper

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We run a Cetrek 619 autopilot course computer with the Cetrek 730-580 compass unit attached to it and of course all other periphery required for its working. This works well.

Recently I bought a second hand Cetrek 930-581 Rate Stepper on ebay, which was sold as improving response and course stability.

When attached to the course computer the rate stepper is recognized by the system and seems to be working, however I cannot find any more information to its working. The rate stepper is attached to the course computer via its dedicated socket/gate, so it does not seem be working with NMEA0183 telegrams (my take on the situation). So far I've not seen a big improvement on the AP behavior (its works extremely well anyway) but I haven't seen the new combination working in a swell with lots of steering needed.

The original AP Installation manual mentions to look into the manual for the rate stepper, which I don't have.

I contacted the company who initially designed the cetrek systems but they have no information on the system anymore (at least that's what they say).

My questions hence are two:
a) anybody out there who either have the manual for the 930-581 and would like to share it?
b) anybody out to there able to tell me what the "rate stepper" actually does? Is s a kind of gyro unit?
Thanks