A professional restorer would be delighted to find these markings. Scribbled  50 years ago by a factory worker; they served as a production note during assembly. The markings have been accidentally preserved by a patina of dirt and petroleum grime. Best I can tell the red painted notation was applied when the tank was stockpiled as it displays upside down as installed. There are other remnant yellow dabs of paint here or there.  A simple brush stroke that indicates that a fitting was checked for tightness or inspected in some way. These original markings will be lost when the tank is rejuvenated with a fresh black protective coat and that is one of the issues with restoration: originality is lost.  It sounds anal, but a dedicated restorer would document these glyphics* and, following the refurbish ,  take steps to re-create them just as they were, thus preserving authenticity.

* guess translation is that ” 128 CA” meant that this fuel tank was intended for a particular application and is shorthand for W128 an internal factory designator for this production series. I can speculate that “CA” is short for Cabriolet.

Progress means stamped numbers and barcodes are applied which can be  scanned and tracked like a parcel  or airline luggage. Just as well — in case we can’t read his writing.