Conceptually similar, there was also the ANS synthesizer [1] designed by the Soviet engineer Evgeny Murzin. Unlike the Mellotron, it was opto-electronic, giving the composer more direct access and control over the "samples". Waveforms were drawn on glass plates, and pressing one of the keys would scan across the associated plate, feeding the generated signal through various filters, etc.
It features prominently in the soundtrack by Eduard Artemyev, for the historical film Siberiade from 1979. [2]