Al, I am thinking the same thing..I think it just corroded away. I have had a small coolant leak since I've installed the FWC..it is entirely possible I had a RWC leak as well...it may explain the inexplicable trace of water I was always seeing under the engine, outside of the motor mounts, that is now green since the FWC conversion.
I also have a slight leak in one of the fittings related to the HX as well, which I was assuming was why I had to add a few ounces of coolant every time I used the boat this summer...now I think it may have been a combination.
Both the manifold & HX are home, so they will get a thorough work-over this winter.
We were invited to a favorite friends' house for New Years today..they asked if we might come by boat..I chuckled and said, "sorry guys, we'll have to drive...half the boat's engine is in the garage.