In your first post you indicate they could be new core plugs. If so, then there would be no way they would match that well.. But looking at them, they sure look original?
Yeah, see..they do look original, but the coreplugs in the side of the block are brass, like you would find in a marine engine...and one of the plugs has a "melt tag" like one that a rebuilder would use for warrantee purposes.
Also, adding to the list of enigmatic trinkets on this engine, is a brass tower bushing...