Leslie 10B / 10B-2S Tremolo Unit for Thomas Electronic Organ

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Don Leslie first offered his invention of the rotating tremolo unit to Laurent Hammond as an accessory for his organs. Hammond rejected the competition with his own loudspeakers. Consequently, Leslie worked with other producers, including Thomas Electronic Organ. The combination of Leslie loudspeakers and Hammond organs was nevertheless practised by musicians. It characterised the Hammond sound.
The Leslie 10B wooden tremolo module was made for the Thomas VL-3A Symphonette Deluxe Electronic Organ, probably in 1961, and is equipped with a Jensen C-8R Loudspeaker. 

The outstanding feature of a Leslie is the rotating sound emitters located in front of the loudspeakers. The purpose of the rotation is to create a vibrato, i.e. a modulation of the pitch by utilising the Doppler effect and the resulting beat. By reflecting off solid walls, several slightly shifted Doppler frequencies are emitted on top of each other, creating a beat effect in addition to the vibrato. (source: Wikipedia)