Fender introduced spring reverb later on with the standalone ... briefly seen in some of the company’s brown Tolex amps from the early ’60s and created using dual-band filtering.
and many refer to this as optical tremolo From the mid-’60s onwards, Fender equipped amps including the Deluxe Reverb, Twin Reverb, Vibroverb, Tremolux, Pro, and others, with a new tremolo circuit.