When I remove the two screws holding the starter together, smelly black oil poured out.  That wasn’t a good sign.  The starter mounts directly the the Triumph engine, which itself has oil inside.  To keep the oil inside the engine, there are two seals.  The first is a simple O-ring around the starter flange, but since oil wasn’t seeping onto the ground, that O-ring performed as expected.  The second is a shaft seal that prevents the engine oil from entering the starter.  Apparently oil got past this seal, but why?  There could be a few possibilities, such as seal failure, damage to the starter shaft, a failed bearing, or excessive crankcase pressure.  Regardless, this inexpensive seal, and the exterior O-ring, will be replaced.

The good news is that the armature tested good – no open or shorted circuits.  The brushes are also in good condition – not surprising given the low mileage of the motorcycle.  I’ll order a new seal and O-ring then put the starter back together and test it.  If it passes the test, it will go on the shelf as a spare part.