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Are they butted?
There are butted ones and bladed ones. I suppose it's not beyond the realms of possibility that either Hoshi or Asahi have, at some time in the past, issued NJS certified plain gauge spokes, so @Husy could research that and if it turns out to be the case he could pay some lavish sum of money to obtain some NOS NJS spokes of a type suitable for shortening, and then void the certification by shortening them.
Are they butted? If not, he could always get them chopped down and rethreaded. Of course, then they wouldn't be NJS...