Some weapons are built to be assembled quickly from innocent-seeming parts
Collapsible weapons always count as “fine” weapons, costing 4 times the usual price, but they do not deal extra damage, and are no harder to break than normal weapons. The extra cost goes entirely into the extra effort of making them.
Collapsible weapons are designed to be taken apart into pieces which each individually seem harmless. These weapons have to be made by skilled craftsmen, and break slightly easier than solid weapons, but can be transported with much less scrutiny.
For example: scattered around a trader’s wagon or flatboat are some lengths of chain, some steel ingots, and some hafts of hardwood. Given a few seconds, however, a flail can be assembled and suddenly brought to bear.