What is that missing instrument on a Beechcraft 1900C? (Full image) Beechcraft 1900C.

I cannot find the name of the missing instrument even in the manuals.
Originally there was no instrument at this place, except for UE series with an electrically driven artificial horizon, where there was a fast erect pushbutton/indicator for the copilot gyro.

According to Wikipedia, UD series is for the military model and UE series is for 1900D, so I assume UC series is for 1900C model. Instrument panel for series UA/UB/UC:


Instrument panel from training manual, models UA/UB/UC (source)

Anything visible here has been added after the aircraft left the plant, when the panel was retrofitted. When browsing pictures of this cockpit online, we can see various additions:


Sources: Left, center, right

Looking at details of your image, we can see there was likely a sticker and a button associated with the instrument. This could fit with a battery temperature indicator and its test button:

  

Source: Right

This configuration was actually found on F-GPYY. Comparison:


Source: Right

Full cockpit of F-GPYY when it was an aircraft of the now defunct French Air Littoral operator:


Photo by Thierry Izzi, source