Well, the car was 37 years with the previous owner and I am pretty sure he did not swap interiors. Rear seats have the same fabric, but also some weather damp on them atm as the car stood outside for a long time.
Maybe its a European spec only? or an option? I am not good with the option codes, but maybe someone is?
Here are some more pics of the tag and interior. Its difficult to get good color in LED inside lighting though..
Edit: I think you are right though: someone else also stated that color was not delivered and must have been the influence of UV!