So Keith and me have been on this fun little CSI project... and I may be way around the bend here, but bear me out.
John's a Mariner. When he isn't playing bass, or watching a Gunners soccer match, he's boating. Or really, the other way around. Penguins, Porpoises, seafaring things... that's him.
I still think what we're seeing is a reinforced headstock repair. But caught in the glare of stage lighting... I see Neptune's Trident on the headstock of #27.