Earliest Marvel Original Art?

Does anyone know what the earliest piece of Marvel original art that exists? When I say Marvel, I mean Fantastic Four #1 and after.

For example, does the original art to Fantastic Four #1 or Amazing Fantasy #15 exist?

There's no reason to believe it doesn't all still exist with the exception of a relative few pages. The earliest Silver Age superhero pages that are commonly known to exist are (I believe - I'm doing this from memory) from FF 3, Avengers 1, X-Men 1, Journey into Mystery 83, Daredevil 1 (and maybe Hulk 1 or 2? certainly 3).

While DC sliced and diced much or most of their 1960s art, Marvel stored it (with the probable exception of their earlier covers, most of which have never surfaced). As you may know, much of Marvel's early artwork was then stolen (mostly in complete story form) around 25 years ago, and one reason some of the more valuable pages don't surface, at least not publicly, is because of questionable propriety.

Have they never found out who the thief or thieves were? Sad to think that original artwork like that has quite possibly been destroyed. Sorta like that French woman who threw all those paintings her son stole into a lake.