Note the listing states this was the first electronic sound synthesizer, but that of course is subjective. There were others. You can find a list by year on 120 Years of Electronic Music.
via this auction


A Helmholtz Sound Synthesizer, manufactured in Chemnitz by Max Kohl after the design by Hermann von Helmholtz, ca 1905.
Estimate 25,000 - 35,000 USD
Wood, brass and steel sound synthesizer, signed " Max Kohl, Chemnitz, 1/8", 39½ x 29", mahogany base fitted with 11 steel tuning forks signed MK, each fork stamped with corresponding note and frequency in vs (vibrations per second, i.e. hertz). Forks 1-10 fixed between pairs of electromagnets and mounted vertically onto wooden platforms (numbered 1-10) along with brass Helmholtz resonators, each pair ranging in size according to their graduating frequencies, each platform with "Aus/Ein" [on/off] switch. 11th fork (marked UT 2 256 vs) mounted horizontally onto wooden platform with electromagnetic coil mounted between tines, adjustable mercury cup below lower tine; one end of board fitted with 2 anodes and 2 cathodes for connection to external 20 volt power sources; other end of board fitted with keyboard of 10 keys (8 ivory, and 2 non-mammalian replacements), each corresponding to 1 tuning fork/resonator pair (UT 2 256 vs; UT 3 512 vs; SOL 3 768 vs; UT 4 1024 vs; MI 4 1280 vs; SOL 4 1536 vs; 1792 vs; UT 5 2048 vs; RE 5 2304 vs; & MI 5 2560 vs). All 11 platforms connected together in series with wire filaments from the horizontal tuning fork."