Usually "marine grade" is more stringent than "aircraft" or "military", at least for corrosion resistance. AN on the bolt spec is Air Force/Navy. But last I checked, all US AN and aircraft bolts were only in US English threading. That might have changed by now, don't know, but the US auto industry went full metric decades ago.

I have a box of new bolts in storage got cheap at a surplus sale and each individually packed with cardboard tube around shaft to prevent nicks, *grade 9*; Usually highest you find here is grade 8.