Rear third brake light for W123s
No, the W123 (sedan, coupe and T) never had a third brake light fittted as standard or as an option.
The third brake light became mandatory in the U.S. in 1986, and by then only a few Ts were being manufactured in Germany. The sedan and coupe had already been discontinued.
That I know of, only the T-series could be ordered from the factory with a set of very peculiar looking third brake lights (a set of two, square in shape) which were mounted on an horizontal chrome bar crudly attached to the sides of the interior rear window. This were mainly provided as an option for customers who ordered the "tow package."
Although, Hella, Bosch and other European companies offer aftermarket third brake lights which fit nicely on the W123.