(Full image) Beechcraft 1900C.
I cannot find the name of the missing instrument even in the manuals.
It could be anything as originally there is no instrument at this place, and actually when browsing pictures of this cockpit online, there are many different items found at this place.
But your detailed image let us assume there was a sticker and a button associated with the instrument:
With these two clues, it can be reasonably assumed the missing instrument is a battery temperature indicator with its test push button. Something like this:
This configuration could be found on F-GPYY, when it was an aircraft of the now defunct French Air Littoral operator. Comparison:
Cockpit of F-GPYY:
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
¹ Pages 6 and 7 of the PDF file.