Frederic Boucher wrote about it:

“Puzzle inspired by the Cover Up created by the great George Sicherman. In my version the 5 pentacubes pieces are identical with holes drilled in each voxel. Even if you manage to completely cover the Y-shaped piece, if you see it through the holes it is not the right solution… This puzzle also has an extra challenge and, only for the PELIKAN version, its own storage box for the pieces, which makes it also a packing puzzle (4 close solutions).”

There are 2 challenges: see description.
The spoiler image is the configuration sent by Pelikan Puzzle (solution of packing problem).
This is definitely a minor spoiler, as the position of the holes on the U-shaped pieces changes on each one. You really have to concentrate to want to find the spoiler. I'm sure you can quickly glance without understanding.