Corrosion as an indicator??
 

I clean crud off the engine every spring and it always looks and works swell after I work on it as part of the spring maintenance... however... I also believe strange appearances can indicate underlying problems: This year I see excessive blackness and crumbly crud at the manifold back/outer/rear surface just before the flange. Does this indicate a particular known problem? And BTW, is that a sacrificial zinc in that area? The usual over-winter crud is at the water jacket plate and a few other areas. Thanks.

The areas you mention are areas where some owners have taken action.
The high end of the exhaust manifold will tend to hold an air bubble and get hot. Some owners have reversed the flow through the manifold. The hose from the thermostat then leads to the transmission end of the manifold, with flow out at the upper end.
As for the cover plate, many owners have changed out the bolted connection and installed studs in the block. Moyer Marine sells a kit for this. I made studs from brass all-thread. An alternate is to install stainless set screws as studs.