WEASEL,
The piece that is screwed into your fuzes is the booster that would detonate the TNT charge in the main body of the projectile. If the booster had been filled with tetryl, it would have a plate inserted into the couterbore that you see, and the edge would have been roll-crimped around to hold the plate in. We also see red and white and green versions of these in the states, with the empty booster. They would be used in inert loaded projectiles.
The piece that is screwed into your fuzes is the booster that would detonate the TNT charge in the main body of the projectile. If the booster had been filled with tetryl, it would have a plate inserted into the couterbore that you see, and the edge would have been roll-crimped around to hold the plate in. We also see red and white and green versions of these in the states, with the empty booster. They would be used in inert loaded projectiles.