Since that day, fans have been talking about the number 9906573 and its possible meaning as it "looks very much like" the serial number on the crate in which the Ark is placed at the end of Raiders of the Lost Ark.

Fellow Indyfans saw a message from Lucasfilm in this number: "the Ark of the Covenant will make a return in Indiana Jones 4."
But in that case, isn't the number 9906573 wrong? As the number that appears on the crate at the end of Raiders of the Lost Ark is 9906753. The 7 and 5 are switched...

What intrigued and confused visitors even more are two mysterious websites; 9906573.com and 9906753.com. Both sites appeared online only days after the release of the new IndianaJones.com. Are these two sites created by Lucasfilm as part of an online marketing campaign?

9906573.com initially contained the release date of Indy 4 with a link to a wikipedia page of the Ark of the Covenant. Later this link got replaced by a reference to a bible text. And now it appears completely empty.

9906753.com shows a close-up image of that number on a crate with a link to the official Indy 4 site and a quote from Raiders: "That thing represents everything we got into archaeology for in the first place"

Well, I've received word from Lucasfilm that they have nothing to do with these two websites.
And about the number on the Indiana Jones 4 teaser poster & website; Lucasfilm says 9906573 is not meant to reflect the Ark of the Covenant at all. It is just another catalog number like the one seen on the crate containing the Ark in Raiders.

So both number sites are definitely made by fans!