1-wind-up toy key. This key will fit the smaller Marx, Schuco, and other German, and Japanese toys. This key that has a 2.75mm opening as stated by the manufacturer, and  measures just under 1/8" square (2.75 MM = 0.28 CM = 0.11 inches = 1/8"). The inside opening is square, and fits on any toy with a square stem. This key will fit into a hole that is just under 1/4" in diameter. We have no way of knowing which specific toys it will fit but if the stem is smaller than 1/8" wide and is square, it has worked for almost every one that has purchased one. Add keys to your windup toys that are missing keys. Put one on your key chain to test toys before you buy them. Again, the opening is approx. 1/8 inch square. This key would be an asset for the toy collector.