Are those stoppers original? I'd assume you dug those and found the stoppers in them, which is why you kept them together. If they are original, it's probably due to the desperation of the war period, and needing something to keep the bottles closed.
Beautiful collection, by the way... Love those colored quarts.
What a great collection! Really amazing - I love the clears, which I don't think I've seen before, and the wide mouth aqua. Is there any in particular that you don't have that you want to add next?