If you are talking about the stainless steel plates that covers the whole sill and hangs over about 1cm, then I believe these came fairly standard on JDM Eunos Roadsters, US delivered \"Miata\", and certain UK delivered MX-5s. All NA's.
NB's had different designs, with black or tan rubber underneath. There are actually a few different OEM scuff plates.
They are hard to come by, and because of that they fetch a bit of a premium.
Hunt around eBay, be patient and persistent, and you can find a bargain. I did.
I bought a set of stick-on ones from Repco only a few days BEFORE I found my bargain on eBay. Aaargh!