It's a 300-piece jigsaw puzzle with completely transparent pieces.
What made it hard:
- No picture.
- The backs and fronts of the pieces are indistinguishable, so each has 8 possible orientations.
- The number of pieces along each edge of the puzzle wasn't given.
What made it easy:
- Every piece with the same configuration of lugs and holes was identical to each other.
- There were three extra pieces, allowing for some flexibility.
- It was a 300-piece puzzle, rather than a 1,000-piece or similar.
In the end, my wife took it over because I was doing it "wrong" so I didn't have much trouble completing it.
Now to get a photograph printed the same size, so I can make it look as if it's a jigsaw puzzle.