This gown was incorrectly preserved by another cleaner in the past. The box used for preservation was the outdated type with a plastic window. It had invisible stains when it was originally process, which oxidized or caramelized over time to cause all the dark yellow stains you see here.
As you can see below the gown brightened up a great deal and most of the stains were completely removed. Now it's on it's way to a proper preservation, which is why the buttons have been removed.