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

It's a missing instrument because it's not part of the standard avionics of the 1900C model but part of the Beechcraft's 1900D aircraft avionics.

If you take a look at pages 0 and 1 of 31-10-00-01 (chapter 31, section 10, unit 00, figure 01) of the Beechcraft 1900D aircraft's IPC (Illustrated Parts Catalog),¹ you'll notice that the hole featured on the picture that you provided is precisely the item 100 (Part Number 101-380031-3), which is the Copilot's Turn and Slip Electric Indicator (technical nomenclature INDICATOR-COPILOT TURN AND SLIP, ELECTRIC).

Looks like this 1900C aircraft is using a 1900D aircraft panel plate/harness but it doesn't have all the 1900D's indicators, hence the missing item 100.

Footnotes:

¹ Pages 6 and 7 of the PDF file.

• That’s no turn and slip indicator – Carlo Felicione Jan 13 '18 at 20:24

I flew the 1900D. We had 20 of them in the fleet but there wasn’t a gauge or hole there.

• Perhaps the difference is because 1900C and 1900D are different? – Mike Brass Sep 14 '19 at 2:09
• For sure they are. I was referring to Yuri’s comment about the photo looking like that 1900C using 1900D panels. Between operators there can be various differences in the avionics package, and ultimately I don’t think there’s anyway for anyone to know simply from the photo what used to be in that spot. There should be a supplement in that aircrafts technical/operating manuals that specifies what instruments/gauges would have been installed. – Ben Sep 17 '19 at 1:03