I have serial numbers on some of my cartridges. The Kiseki Blue NS I bought from Kron doesn't have one. I did a Google image search and couldn't find an image of one with a serial number.
Generally speaking, if you buy an "international" version of something, it's not going to come with a US warranty. As long as the seller has a lot of positive reviews and you have a level of comfort that it's an authentic item, I think your risk level is relatively low.
Generally speaking, if you buy an "international" version of something, it's not going to come with a US warranty. As long as the seller has a lot of positive reviews and you have a level of comfort that it's an authentic item, I think your risk level is relatively low.