What are these puzzle pieces for?

Problem statement

Here are 13 items, with the transparent plastic box they were in, bought together in a charity shop [US: thrift]. There was no documentation with them. What is the puzzle, and what is its solution?

Each of the pieces has been made by gluing together plastic cubes. Eleven are identical. They fit snugly into the box, filling it.

Hint to metapuzzle: show

Solution: show

This is one of several pages on puzzles and metapuzzles .