Most of those old army "Down" bags are filled with feathers and maybe some down. It will insulate, but you will get no weight advantage and not much compression advantage from such a mix. Free is good, but unless your bag is different from all the other "old Army" bags I've seen, it is not such a great deal in the end. As a test- just grab a hand full of the bag with some feathers and squeeze. You shouldn't feel any stiff feather shafts or sharp pointy things inside. If you do then you have feathers.
"Do what you can, with what you have, where you are."-Theodore Roosevelt