It's always good to find original notes or paperwork when stripping things apart, i found a dated paper inside my interior mirror when i had the mirror re-silvered
There's no reason to think your padding isn't original, horsehair or other natural fibres can last centuries, there are plenty of 18th and 19th century chairs and settees still in use with the original padding, even after being recovered
I've read that Fehrer supplied the pads after Emil Hauenschild stopped trading, but can't see what date that happened, possibly with the 280SL, but only an owner who's stripped their seats or have original seats could confirm this