The very first spring reverbs were produced by Hammond for their organs in the 1950s.
 
Starting in 1961, Fender made there own Reverbation units using a spring unit bought from Hammond.
 
Sold as an external effect unit, it has been first included in a combo amp with the Fender Vibroverb in 1963 but was still available for sale as a separate effect until 1966 (then stopped and again available in 1976 until 1978).
 
This 1963 Fender Reverb unit has the XXX circuit
 
Specs :
Dimension : 10″ x 19″ x 7½” (25.4 x 48.3 x 19.1 cm)
Weight: 16 lbs. (7.3 Kg)
Input/output : In, Out & Footswitch connector.
Settings : Dwell, Mixer, Tone
Serial : R04305