I noticed when I stripped down my seats (Dark Red Leather) they had matching numbers ( 3 digits) probably Grease Pencil on each piece of leather. (Which I saved for posterity).
Also when stripping down the Dashboard the same numbers appeared on the backside of each piece also the door panels.
this way I know it is all original from the factory.
they could be elsewhere but I have not seen any as of yet.
just for information for those who never stripped out major parts of there SL's.
Mine is a 1971 280Sl (Mfg 2/71)
Tom M