Heavy Duty 3-Wire Surveillance Kit: Features acoustic tube earphone with TWIST CONNECTOR, remote PTT switch and low-profile lapel microphone. Straight cable.Heavy Duty 3 Wire Surveillance Kit: Features acoustic tube earphone with TWIST CONNECTOR, remote PTT switch and low profile lapel microphone. Straight cable. The SPM 3300 Series is a heavy duty 3 wire surveillance kit. The kit features a clear acoustic tube earphone with TWIST CONNECTOR for easy removal, plus a small, covert microphone which can be clipped to the user's lapel or shirt sleeve, and a separate barrel style PTT which is held in the hand.