History:

Electric saxophones in the 1960s emerged featuring early amplification and electronic effects. Key developments included the 1967 Selmer Varitone system, which allowed players such as Eddie Harris and Sonny Stitt to add octave and vibrato effects.

Vox and Arbiter produced their own in the UK & Europe and Maestro in the U.S.

https://mbensonsax.wordpress.com/2015/09/13/the-life-and-death-of-the-electric-saxophone/ 

https://www.jazztimes.com/features/columns/the-art-of-electric-sax/?v=5435c69ed3bc