The term "vibrato unit" was introduced on high-end Fender guitar amplifiers in the 1950s, starting with the Vibrolux amplifier in 1956, in the same period in which what is now called a "tremolo arm" was introduced on Fender guitars.
In both the case of the "tremolo arm" and "vibrato unit", Leo Fender had reversed the established usage of the terms vibrato and tremolo.