There was also a system in JPs whereby the aircraft would transmit a beep if the gear was down (UPI?).
Yes, UUPIs (Ultrasonic Undercarriage Position Indicator) gave three beeps on the tower frequency when the pilot transmitted with U/C selected down, and was I believe the invention of Mr Jeff Jefferson, a civilian Electronics Engineer employed at RAF Linton-On-Ouse. As an additional safeguard and indication for the Caravan Controller, JPs had a small white light on the aircraft nose that flashed continously when the gear was down. The SOP at night for JP pilots was to ensure that the nose mounted landing light was OFF when on final(s), otherwise the U/C light could not be seen.