"Kyutai Kansetsu Sutokkingu"(Spherical Joint Stocking) is a coterie stocking sold at Bungaku Furima(=literature flea-market), a dojinshi sale dedicated for literature-related things only, by circle Ojosama Gakkou Shojo Bu(preppie school girls section). The stocking has globe joint painted on knees, to make your leg like real figure.
The stockings, 2,000 yen(US$25) seems sold out on their online shop, currently on order.
But why? I guess some people might love figures too much so that now they want to become like that. It is interesting because those joints originally showed their incompleteness of mimicking human beings.
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