That's the park sprag cable. Its job is to engage or disengage the park sprag when the driver shifts into or out of Park.
392Mopar: that filter looks nice and red, but it's also the wrong one. It's the big '66-up filter with only one fluid intake port on top. If one of these is run on a '65 or earlier Torqueflite without cutting the necessary 2nd fluid intake port out of the upper surface of the filter, the rear pump will starve, whine, and eventually spall.