Another mystery of the Bunker-42

This rectangular section is called tubing. They are made of a special kind of cast iron. The weight of each is about 1 ton! 16 these parts form a cast of the ring, and the rings linked to each other in the tunnel. It was built and the Bunker-42 and the Moscow metro. Through a hole in the center finish especially strong concrete.

The abbreviation SNL stands for station normal lightweight. Also, the year of production should be specified, but instead on the hopper tubes arbitrary numbers. Their meaning is one of the mysteries of the bunker...