Nice example of an early Shimano indexing road rear derailleur, the 600 SIS, model # RD-6208.  Indexes with appropriate shifters on six speed Shimano-spaced freewheels and cassettes.  Also works great as a friction shifting derailleur.  Mechanically sound, with no play or wiggle in pivots or cage.  Alignment appears spot on.  Cosmetically, many scratches, mostly minor, but no discernible rust.  Upper pulley is weirdly scratched, especially along sides, indicating that previous owner had issues with chains, but the tooth profile is relatively fresh.  Please study the pictures, and ask any questions.  Thanks for reading!