The New York Times article Boeing Believed a 737 Max Warning Light Was Standard. It Wasn’t. includes the following:
When Boeing began delivering its 737 Max to customers in 2017, the company believed that a key cockpit warning light was a standard feature in all of the new jets.
But months after the planes were flying, company engineers realized that the warning light worked only on planes whose customers had bought a different, optional indicator.
The warning light notifies pilots of a disagreement in the sensors that measure which direction the plane is pointed, a potential sign of a malfunction. This light could have provided critical information to the pilots on two flights that crashed shortly after takeoff in recent months.
Because only 20 percent of customers had purchased the optional indicator, the warning light was not working on most of Boeing’s new jets. Neither Lion Air nor Ethiopian had the indicator.
note: All instances of boldface have been added for emphasis.
My question is asked to clarify the nature and appearance of the warning light itself.
Is this a physical light bulb-based indicator on a physical panel, or is it graphically implemented within a flat panel display screen? In either case, if the option described above was not purchased, is the (now non-functional) warning light still present and visible?
In that case, is there any secondary visual indication that would allow one to know that it's not working?