Thanks for all the info. Apologies for the lack of response, but I've been a bit under the weather.
I did manage to find the ISO-Core UltraClear datasheet, and it states it's REL0. It also has this info (which is what tooki referenced?):
Patents
* FELDER GMBH produces all NiGe-alloys with the licence for Fuji patents (JP 3296289, USP 6.179.935 B1 and DE 198 16 671 C2) as well as for Nihon Superior patents (DE 69918758 and EP 0985486).
I know Nihon Superior came up with SN100C. Would that imply that Sn100Ni+ is a Fuji product?
Speaking of, I haven't found Felder's SN100C available for sale
anywhere. Given how similar they both are, I'm guessing there's very little, if any, practical difference between the two. So no big loss.
But finding a supplier for their Sn100Ni+ is also a struggle. The only two options I've found so far are Reichelt (with expensive international shipping), and the local branch of RS (which tacks on a fee if one doesn't meet quite a substantial minimum order).