I think the point regarding the Metropolis US one sheet is that not a single copy has ever been found and so collectors of these items would go mental if one was found.
This is true that one hasn't come up before, and I think that is why something like the Frankenstein insert (also previously unknown) went for such a crazy price even though it was fairly heavily restored. But other unique pieces have come to auction before and sometimes spectacularly failed, like Todd's Bride of Frankenstein one-sheet, and even when Bruce tried to auction the only known Frankenstein teaser one-sheet it failed to meet a much more reasonable reserve ($250K if I remember correctly, and this was after a regular one-sheet had sold for $198K and the Mummy had "sold" for $451K - now that is an actual suspicious sale.)
The strong price Heritage got for the lesser style Kong 3-sheet is a step in the right direction, but it remains to be seen if that purchaser has an appetite for other titles as well (and if more competition will step up to challenge him).
Well the other dynamic is that some of the other candidates for "First Million Dollar Poster" ...like the fishler frankie 6 sheet ...and a singular Metropolis 3 sheet...have never been auctioned by themselves in a major auction yet..they were private sales and the metropolis poster was part of a group ....not to mention if something like a mummy 6 sheet or 3 sheet was discovered...in a better market ...who knows?!?!
Actually the Metropolis 3-sheet has been auctioned before, and the last time it was auctioned it was several years after the "record" $451K paid for the Mummy 1-sheet. And it got a single bid at the minimum of $200K. And of course there is a beautiful Mummy 3-sheet known and it was offered around a few years back to the key players for a price under $500K and a buyer could not be found. If it went for public auction, who knows? But again, as beautiful as a piece that it is (and it is simply amazing in person) it is hard to explain to the general public why a poster for a "lesser" title should be a million dollar item. That is why I keep coming back to the King Kong 3-sheet - it is something that is easy to understand why it "should" be a million dollar poster.
I had a really wise dealer friend once tell me that if it takes more than a sentence or two to explain to the public why an item should be valued at what it is, then it is really hard for the item to hold it's value: First appearance of Superman, Mickey Mantle rookie, James Bond's Aston Martin, Dorothy's Ruby Slippers, etc.
Mummy 3-sheet just doesn't have the same ring to it.
I know of one who has about 500 lobby cards for the 20'-30's which he purchased before many on this forum were even born and no one knows what's in his collection, but he has allowed me to see the boxes they are kept in and pulling out random cards, there were some really choice ones!
Speaking of choice early lobbies - did anyone see the incredible selection in the upcoming Heritage auction? Tons of amazing things that haven't come up for auction in decades, if at all before.