Well That’s a Fine Start

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Google turned up this post from 2016 which had it seems a similar issue that did not have the repair documented so we don't know what happened ;

[url=http://cb1100forum.com/forum/showthread.php?tid=8487&page=2]http://cb1100forum.com/forum/showthread....487&page=2

Other than that a few plugs have o-rings, they have a groove close to the flange to hold the o-ring, another ( sideways view ) picture of the plug would help to find out if this is the case and if sealer has been applied and how much unless someone knows the answer.

However the dynamic balancer shaft ( just to the right with the clamping bolt) is very close by and it has a seal that weeps very slightly sometimes, worth a second look before taking the galley plug out.