My, what a wonderfully strange creation! I think the seller is correct that it was intended as a reverberation device. It may have served as the tone cabinet for an electronic organ in a church; it's styling looks the part anyway. The audio signal (whatever it was) was fed to the 600B, which spoke and excited the strings tuned to many different pitches and created a haunting sustained sound. The walls of what at first glance appeared to be a bass horn are thin, and with the S holes allow communication with the air volumes trapped behind, adding to the reverberant effect. It looks to me like someone added the red cloth later on to reduce the output, so it may have worked even better than intended.

If one depresses the sustain pedal on a normal piano and shouts into the instrument it is easy to hear the sort of effect that the designer of this device was after. The tuning pins and wire appear to be standard piano items, possibly obtained from the church's piano tuner. If it wasn't in PA I'd be tempted to pick it up and experiment with it.