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What do you mean by "outer" vents? the assymetrical ones on the ends on the top of the dash?
What do you mean by "inboard" vents also?
The only ones I can see are the four eye-balls, the ones at the very front of the top of the dash that directs air onto the windscreen.

I read ages ago that to remove eyeball vents, you thread string through and give it a sharp pull. And AFAIK, the 'rings' aren't a separate trim piece so not removable.

Aero-next year- just ring or pop down to the MX5 factory - they will tell you. Not sure if the Haynes manual has the info- my local library has it. Not sure if you can get a look with an inspection mirror.

You need to pop an old credit card down either side of the vent to unclip it. The yank it out using the string.