Did you as the original owner, order the car, or just buy it at the dealership because they had it in stock?

There were custom variations allowed to be ordered. Have a look at the tables at the end of this link

https://www.w126-wiki.de/index.php?title=Lackierung_und_Polster

You will find that for your dark blue paint, there were 3 standard interior color options, (x) and 2 optional/possible (o) interiors Dattle and Mittelrot. The optional colors were not added to the data plate at build time (Black/Limo) usually being the default. I've seen that before on other custom order MBs of the era that received cloth, velour interiors etc, as US spec cars otherwise.

My understanding is that the optional interior colors were custom order options. Maybe yours was custom ordered, and then not purchased by the ordering customer and offered for sale at the dealership instead.

In either event 134 Palomino/Dattel was available, and if that is what you have, that is your color code for the interior.