A breechblock is a metal shutter that closes the breech (where the projectile is loaded) of a gun when it fires. A field gun is a large gun or cannon that can be moved around the battle field. Mr Verloc’s career as a secret agent began during his time in the French artillery, a combat unit that uses cannons and field guns. One of his tasks was to obtain the design of this part of the gun to give an advantage to the enemy side. The reference is slightly anachronistic. The French artillery brought out a new field gun (the canon de modele 1897) in 1898 after five years of secret tests and trials. Conrad may have had this in his mind when thinking about Verloc's early career.