It's probably authentic, the part is only around $125 new.
On the NSX-R they install a jam-nut below the shift boot, which keeps the boot from sinking too low on the stalk; if the installer didn't use a jam nut the stalk will be exposed, which isn't aesthetically pleasing.
You can buy the nut from Acura for around $3, it's PN 90310-SL0-Z00. :smile: