As you have already figured out this is a Benmar Power unit. Maybe one of the best rudder
drive units ever manufactured. The unit on the side is probably some type of electric
drive for the clutch
. Pull the back cover off and you will find a printed circuit card with the motor
plugged into it. Un connect the motor and apply 12V directly to the motor leads. Should drive in both directions (reverse the polarity). Keep your fingers out of the way.
I have been running one for a number of years and driving it with an assortment of Raymarine
AP units (currently using an X-5 /6002. Just connect the AP motor output drive and toss the Benmar electrics. Mine is set up with a simple mechanical clutching arrangement. If it is functioning OK then pull the unit and clean and relube the gears (metal gears and oillite bearings). open the motor, blow out the dust, and ad a little warm oil
to the motor ball bearings.
If you can't get it to work
I have 3 spares. I spent 30 years designing automatic control systems for the US Navy
. IMO there is not a better solution to a rack and pinion rudder