Hi,
When I flew on the 300/500 only a few aircrafts had it, on the F/O side.
Directly from B733/735/WL FCOM:
SPEED BRAKE Caution Light (as installed)
Flashing (amber) – indicates air/ground sensor in air position, SPEED BRAKE lever aft of ARMED position and flaps extended beyond position 10.
I hope You will find it helpful ☺
Ciao 😉