Here we have a lovely Vintage, 1980s, Simplex SX610 Second Generation Rear Derailleur. The ribbed body indicates it’s a second generation with date on manufacture likely to be 1983-85.
The SX610 is a friction-only derailleur designed to work with 5, 6, or 7-speed setups and was a popular touring derailleur in the 1980s.
The mech in overall excellent condition and works smoothly. The adjustment bolts turn fine and there is no rust.
Prior to sale it’s been cleaned and degreased and is now ready to fit.
It would make a great addition to a vintage bike build project.