Koen:
I live in Germany. I acquired my 1987 MkIV (AK1200CH) earlier this year. It is fitted with a 302EFI and has a controlled catalytic converter which is known here as 'G-Kat' ('geregelter Katalysator'). I did not have problems with getting the green environment badge.
I was advised, however, that without the G-Kat my car would not have been eligible for the green badge. If AK1240 has a G-Kat this will be reflected in the registration papers. There are various sites in Germany where you can obtain the green badge if you show them registration papers confirming that the car is fitted with a G-Kat. I got my badge directly from the local motor vehicles registration office when I registered my car. Other bodies that issue green badges include TÜV and DEKRA (equivalents, in this respect, of the MOT in UK), automobile clubs such as the ADAC, and car repair shops.
Jürgen