Muck, debris, or maybe even rust - hard to say with these old discolored cast iron rings. Didn't want to use anything that might remove metal, so rubbed them lightly with Scotchbrite. Ran a dab of it through the groove and it's all good now. Just another sign of the neglect and abuse this engine has suffered. The jugs are on now and the next step us buttoning up the bottom of #2 rod, then torquing the case base nuts and starting head mounting with the freshly annealed copper gasket and Hondabond 4 around the dowels. Then the rocker boxes, they might need some lapping on their base. Have Covseal gaskets for them. And I'm pretty sure the covers are going to need helicoils for the cover bolts.
Of course, along with all of that getting the PRTs to seal (plenty of write-ups on that-it seldom goes easy) and positioning the rods properly.
Looking forward to the GL1000. I feel more comfortable with them, and they are a much more robust machine with superior engineering and manufacture. Just make sure you only use JIS screwdrivers!