A replacement battery cover for the Atari Lynx II.
I've seen a few atari lynx' on ebay that have no battery cover - the covers seem to fall down the back of the sofa and get lost, I bought one and wanted it to feel whole again.
This model is missing the foam insert (a neoprene closed cell foam sheet about 2mm thick). This can be bought easily from HobbyCraft or other craft stores. Note that the colour-matching isn't perfect, the black is not grey and the white is certainly not grey, nor are any of the other colours. The black looks surprisingly good, but wouldn't 'fool' people. You could paint the part if you were super serious, or just buy it in 'hot pink' and embrace the novelty.