I really don’t know how many pages were on our old websites but I mas sure there are hundreds of it. In the mean time we changed our old product names and that’s why we generated new URLs according to new product names. We did this for twice or thrice in last 4-5 years. That’s why I am sure there are junks of such old pages which are still in Google's index which need to be searched and then assigned accurate HTTP header statuses.