Re: 55 hp mystery
OH! If you mean nthe large hole visible at the rear of the pump mounting: On earlier engines, the water pick-up was behind the exhaust snout. There was a recess cast into the cavitation plate from the snout to the back of the gearcase. A perforated metal plate covered the recess and made a water channel. A TUBE ran from behind the prop up to the hole you see under the water pump. It seems to me that right now, without that tube, at anything over the speed where the pump goes above the outside waterline, it is going to suck air and the engine will overheat.
The theory was that water pressure off the prop would help push water to the pump. Never a really good system but it did work. I don't know When Chrysler changed it to a pick-up just above the gearcase torpedo.
If you mean what appears to be a smaller vertical hole inside the larger one--I can't help. Not yet--Let me take a look at my gearcases tomorrow.
