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Looking at the skiis it appears that they first produced the "boot" that fits on the tire, then realised that they could be improved by adding more surface area to the ski bottom so they bolted a wooden ski to the bottom of the boot. There appear to have been two different wooden skiis produced, the ones mounted to the boots in the picture have more squared ends than the others, and the bends in them are different.In the picture below you may be able to see the runners still on the bottom of the "boot"
