This listing sheds some light on this unmarked item. The photos show that printed on the top flap is the exhortation to "Buy Bonds." This drives the seller's date attribution to the 1940s, something with which I do not disagree. The one in the collection does not have the reference to bonds, affirming my attribution to the 1950s. Interestingly, both variants have "Patent Pending" printed on the bottom.