"CNPnnnH" was a Worcester mark issued from April 1970 onwards. As yours isn't in the DVLA system then the two closest either side numerically is the best you can hope for.
Why not apply to MMC for the chassis record which will tell you the date the car was dispatched from the factory. I think is probably a better "birthday" to celebrate anyway. You can order it from the website, a bit pricey at £75 but it will answer the question.
