http://www.japan-partner.com/check-manufacture-year.php
This website tells you when your car was first manufactured. I found it while trying to sort out my first registration date with DVLA/VOSA.
Simply put in the manufacturer and chassis number and it tells you when your car was manufactured. It’s accurate as I have had confirmed it by calling Mazda and giving them my chassis number.
Edited to add a comment by SAZ 99611
Sometime ago, I put together a Roadster VIN spreadsheet:
http://hagar.zoomshare.com/files/roadster_vins.xls
Based on some known Roadster registration dates and official Mazda production numbers. My spreadsheet put my car in as a early Feb 1996, so not bad.
I did say at the time it will probably need tweaking, based on real data from owners.
Interestingly, the oldest records it holds for the Roadster is for NA6CE100020, built 9th June 1989. All the published records say ~21, a S-Pack 5-speed, was first down the Roadster line.
I will make appropriate changes to the spreadsheet.