Seats w/ headrest speakers that fit in a Miata are likely OEM seats. Headrest speakers were standard on the '91 BRG and available all years '90 through '97.
Two-tone upholstery is aftermarket - like Katzkins and others that are available from typical Miata vendors. Some had it dealer installed, though that would seem a little odd for a BRG that would have had leather from the factory.
There are 2 bottom cushion shapes in NA seats. Early seats - through about '95 appear to have a seat cushion with separate side bolsters. The later cars look more like a single cushion with integrated side bolsters.
Hope this helps.