So the pictures seem to show that it shifts an actual clutch, like a regular FNR cluch dog, that slide back and forth on the prop shaft, with a cam that moves up and down. The cam/plunger is sprung down, but is supposed to lift up with the cable.. It looks like the whole thing should just pull vertically up and out, once the gear case is removed from the leg. #19 seems to be the retainer. Maybe you can pry up on the cam, through the prop shaft hole?
Long live American manufacturing!
