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According to Jack's official site "In 1976, Jack moved to the fretless bass. After stints with some Aria and Spector long-scale basses, he found Warwick, a German producer of high quality electric basses. Jack made some suggestions for improving the balance and pickups, and Warwick produced the bass that served thereafter as his main instrument. The Warwick Jack Bruce Signature Model is a modified fretless 4-string Thumb Bass featuring MEC active pickups and LED position markers on the side of the neck."
The source and notes on this submission are correct.
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