Just took a look. Would be a PITA to set up for a photo, because my tank is on the bike with fuel in it, and I'm going for a test ride.
Looks like a rivet nut to me. I'm not sure how they sealed it at the factory. I'm not a restoration guy. They were all good in the tank I have, so never had to figure it out. One of the restoration gurus should be able to provide more info. Try google for sealing rivet nuts in the meantime. A good coat of tank sealer might be able to seal around rivet nut.