The Tektronix 7J20 is an optical spectrometer for the 250 nm to 1100 nm region, based on a Czerny-Turner Monochromator. It only appears in the 1975 catalog.
It is a "rapid scan" spectrometer, which means that it can capture the spectrum quickly enough to show how the spectrum of an optical input changes at temporal resolution of a few milliseconds.
The 7J20 is very rare. According to Dennis Tillman, Best estimate is ~40 were actually built.