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Sorry don't have an answer but if it were me I'd assume the one with your actual engine code in the part number would be the safe bet. BP2Y is that an overseas version of the engines?
Doing a quick search on BP2Y it comes up with tons of parts from valve train to TPS but when looking at vehicle compatibility it lists a bunch of different models and even some different makes like toyota and kia. Maybe it's just a part number not specific to a mazda miata they all share?
Yeah, I've had the same thought, but I don't want to assume on an engine build. Manufacturers share parts all over the place.
I suspect that the part number changed during the mid 2000 model year.
That part number change would then match the change in valve design between the BP4W and BP6D, a noticeable difference in airflow according to my builder, why my VVT stock valves now live on in some other head that needed to have Mazda OEM valves".
Caveat, EU market, since you often seem to get artificially poor parts.