Puzzle from 1965 Soviet Magazine

Юный Техник 1965 Issue 9 Page 10

Cube Puzzle

The game "Cube Puzzle for All" is well known to many children. The task is to build a set of puzzle-figures from seven block elements as shown on the picture. In total there are 27 smaller cubes. There are 11 thousand ways to assemble the simplest of these figures - a cube, but it can be hard to do for many.

Try to find mathematically to which of seven elements would belong the small cube in the center of the assembled cube. It does not depend on how you assembled it.
A. Goryachev