The Sendai Museum exhibits a potentially original early Kastane blade see http://www.vikingsword.com/vb/showthread.php?t=14998
where the weapon is discussed in depth...collected intact, by chance in the Philipines, by the Japanese diplomat and Samurai Hasekura Tsunenaga and this may be backed up by the stone carving with identical form below and expertly presented at post #112.
The Storta style of blade whilst intriguing, puzzling and stunning!! may have been introduced earlier, though, that is pure conjecture but in line with the hypothesis on European blade diffusion / the Moors /etc.
The interesting reverse theory that this shape of blade may in fact be of the early home grown form and occasionally favoured by warriors without the Vajra Quillons perhaps so that the index finger could be looped over the guard for added purchase power on the thick backblade? Perhaps quillons were optional?
(Though it is not without possibility that this was simply artistic stone carvers artistic impression the other details in the carving seem very accurate)
Ibrahiim al Balooshi.