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These screws are a bit of a wild goose hunt. They only apply to the expansion card and they are double set screws. By that I mean, the first part of the holds the expansion card to the case and the second part holds the expansion card together. But we'll get to that later.
The top of the main case is held in place by tabs. These tabs (red arrows) are released with a gentle twist of a flat-head screwdriver.
The layout is very utilitarian - making judicious use of expansion slots.
It is also very dusty - our poor PC Jr. has obviously been neglected. But it still works - they don't make'em like they used to I suppose.