John, Is this the round you are talking about? If so this is actually made by the Cobra Gun Company in the UK. The samples that have shown up have IMI headstamps because that was the cases they used for that batch of cartridges. Most of those sold have no headstamp. I have spoken to the gentleman who holds the patent on this cartridge who is known to a number of the members of the Forum. Completely legit item.
If you are talking about a different IMI load, I don’t know if it, which puts it in a very large group of 9mms.
PS: Looking at this round again, it likely would have worked just as well with phonograph needles, and since the Fakes above appear to have a single cup filled with needles , they likely would have worked as well as this Cobra load (which was not designed as a flechette, but as a prefragmented bullet). A case of science following art!