score:5
The requirements you are quoting are simply wrong.
Petrol powered cars will either get a green environment sticker or no sticker at all. With very few exceptions, the only requirement is that the car is equipped with a three-way catalytic converter (TWC). A TWC became mandatory in many European countries in the late 80ies. In Germany, the requirement was not introduced until January 1st 1993. Maybe that is the source of the confusion. All petrol cars registered in Germany on or after January 1st 1993 are automatically entitled to a green environment sticker, because the TWC required for a green sticker became mandatory for a registration on or after this date.
That does however not mean, that a registration before that date excludes the car from being issued a green sticker. A TWC had been required in many other countries long before the requirement was introduced in Germany and it is not uncommon that cars produced before 1993 are equipped with a TWC and fulfil the requirements for a green sticker. All variants of the Porsche 964 were equipped with a TWC and are eligible for the green sticker.
If ANWB refuses to issue a sticker based on false premises, there are several options for you to order the sticker from Germany. One option is TÜV Süd, which sells the sticker for 15€ incl. postage. You have to provide a scan of your registration certificate when ordering the sticker. You could also visit a TÜV Süd office on your way to Stuttgart and get the sticker for 6€.