I don't think you wil ever get down to the date level unless you get lucky and find some inspection paperwork from the factory tucked in some nook in the car. Also, I think the VIN plate on the car will have the production month stamped on it.
Good call! I totally forgot about the production month on the VIN plate! Maybe I'll make a judgement call and use the date we took delivery of it [have the original bill of sale my dad signed] and the month of production. Sort of a hybrid birthday.
You just know the answer to this is in some archive in Japan...too bad there's really no way to get at it