A2S4000 & A2S4100
Apple IIc Bottom Case Shield. Metal shield with insulation paper. It is not meant to be removable so would have to be glued on to the case.
At some point as the computer changed from A2S4000 to A2S4100, the bottom case was coated on the inside with black and they did away with this shield. So if your IIc is black inside, you don't need it.