One of the stranger objects I had stashed are some screw-like objects with a fat smooth cylinder in the middle of the screw. So they're not cylinder-head screws, because there's thread before and after the cylinder part, which is considerably thicker than the surrounding screw. So what the hell are those things for? What are they called? I guess they're some kind of screw-hinge device?

[SM] Sounds like anchor screws used for attaching curtain rail holders. One side screws into the wall, the other into the rail holder. If they are beefy then they might be used for attaching sinks and other similar items. If they are massive, then maybe for holding railway rails to the sleepers. All these types have different threads on each end.

Some sort of curtain fastening arrangement seems to be the best explanation. I think I'll call that a win.

*Contentious statement alert* Most unidentified objects in abandoned drawers and neglected biscuit tins of 'things' are to do with hanging curtains. Curtains are the most complicated of household artefacts and no two systems are interchangeable or compatible.

[pen] Ha! You've obviously never had to deal with shelving units...