The Robert Johnson Gibson L-1 was offered for sale about a decade ago for a cool $6m and had interested parties. Sadly, the provenance essentially turned out to be a letter that read 'we really, really, really, really want this to be the guitar in the Robert Johnson photo', at which point several very heavy wallets snapped swiftly closed. It's an interesting possibility though, if an L-1 surfaced with a convincing provenance to connect it to Johnson. The other missing in action Gibson guitar that I'm sure would cruise well into seven figures is the re-topped Nick Lucas Bob Dylan used for the Don't Look Back era. Being in mind Dylan already has a million dollar guitar under his belt and the Nick Lucas is surely the most well known Dylan guitar I think that one could be a big one.