Just noticed this post.
There is a story that the 'brass' (actually bronze) breech blocks were made when Paris was besieged and were cast from church bells as they could not get the iron forgings. The truth is that, following Sedan in the 1870 War, the French National Government was desperate for arms and sought production from smaller manufacturers. Some were too small to have the forging ability to make forged iron breech blocks so they cast them in bronze. Looks pretty, and adequate if not abused, but not as strong as the iron ones and more likely to stretch if overpowered. The same reason was behind the Confederate bronze frames for copy Colt revolvers so beloved of modern Italian reproducers which have the same weakness of stretching with strong loads.