i'd be afraid to put a whole quilt of that age into a tub of water unless i knew it had already been washed at least a few times. one or more of the colored fabrics might run.
instead, i'd use a Stain Stick, Tide Pen, or similar product and apply it only to the stained area. rinse only where you applied.
i used oxyclean to remove similar mystery stains from a friend's ancient quilt. use it only if the tide doesn't work. use the powdered kind and make a paste. apply it carefully and only to the stained area. unless you know the colors won't run, be careful to rinse only the pasted spots.
good luck. that's a beautiful quilt. :P