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I might be getting this wrong but I dont think I'm far off. In the early days of Roadster racing Mazdaspeed built and sold a few roadsters for a road-spec roadster race series. As far as I can tell they didn't really have much past bolt-ons however they were looked after and had a ton of extra care at the factory. Once the series ended the cars were just put on the road like any other as they had all the legal paper work required. I really dont know if they had any info stamped on them beyond the cam cover for all I know he could have fitted this years ago to an otherwise normal roadster.
As for value I really dont know. All the value comes from the badges and paper work. If you love to own historically interesting 5's go for it otherwise it's just like any other.
Saying that if I were shopping I'd take interest. Most people wont care or know but to the right people this is a real gem and should be taken care of.
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Back in the 90's MazdaSpeed sold HP kits for the roadster over the counter. A spec and B spec kits, which consisted of different things; heads, ecu's throttle bodies, etc. So it might be a MS head on the motor, or just the cam cover. It would be a safer bet if it was documented.