I'm surprised to hear that the original leather may have been vegetable tanned. Chrome tanning was discovered in the 19th century over 100 years before the W113 chassis and only takes about ONE day to dye, where vegetable tanning takes 100 days.
Vegetable tanning is most commonly used for soles of shoes, belts and straps.
The tannins only account for part of the car's leather substance, not the whole thing.