Suppress Certain Overtones In A Musical Instrument's Sound Wave
That piece of hardware is a bandstop filter. It removes one overtone and is usable maximally at one octave pitch range if it's not a digital signal processor with an adaptive filtering program, but a fixed filter circuit.

For that 1 octave range the filter attenuates one octave in the overtone range. Accurate results need quite complex filter (=high order transfer function). 

A  bandstop filter is common in mixing desks as parametric eq or graphic  eq. which can be used to attenuate some frequency band. In modern desks they really are sharp enough for attenuating a predefined overtone, but they are actually programmed dsp devices,  not filter circuits.

Those equalizers are also available as rack devices both as dsp and filter circuits

Premium processing software for musicians exist in wide varieties. Their capablities go far beyond any fixed filtering circuits. The software is available as well for realtime performances as for postprocessing the recordings.

ADDENDUM: for demonstration purposes you should consider to play only recordings to be compared. one unprocessed and one filtered, because


you cannot practically remove the overtones from the direct acoustic sound of the instrument, only from the electric signal.
no recording can have the richness and fidelity of the original acoustic sound in proper acoustics


Nearly all pieces of Audio workstation software have the needed filter included and more is available as plugins. Premium filter plugins such as FabFilter Pro-Q are well worth to check to see what is available and possible to use by the musicians, not mathematicians. Free 1 to 4 week trials are generally available.