It's more likely that the original stain was from the quilt being stored on wooden shelves or in wooden drawers, and not rust at all. The stain color is very similar, but for rust, you'd need both metal and water to be present and how often does that happen in quilt storage? Besides, diamondee is correct, chlorine bleach sets rust stains, it doesn't remove them.
Cream of tartar is more frequently mixed with hydrogen peroxide (or sometimes lemon juice) to remove stains, rust included, from fabric than it is with bleach. It's one of those vintage household remedies.
Last edited by ghostrider; 08-14-2014 at 10:10 AM.