This plate type is know by a few names – sheffield, clad, bi-metal, cold rolled. It was the main form of silver plating before electroplating was invented. A simple explanation of the process would be a sheet of silver is placed on a copper ingot historically with some borax and is rolled under pressure in a cold rolling mill. This is done several times with the metal being annealed in a annealing oven between each pass through the cold rolling mill to prevent it cracking. The silver is contact bonded to the copper rather than being melted on. Its not always easy to tell if a vintage plate was clad or electroplated (both existed in the daguerreian era) as clad plates were also galvanized prior to use – a fine layer of extra silver was made by placing the plate in a silver chloride solution, the combination of the silver and copper creates a weak electrical current which causes silver deposition in the solution on the silver side and etches the copper side of the plate. The end result is that there is a silver layer on the back of the plate as with fully electroplated plates
The hallmark “40” tells of the thickness of the layer of silver in this plate – the ratio of silver to copper is 1:40, plates 1:20 were also used and provided more silver for plates in rebuffing/reusing before the copper layer shows through.
Vintage plates like this that have been through the wars of handling often are warped/not flat. This makes it difficult to reuse them as you will find you get uneven buffing results. Personally I don’t reuse vintage plates regardless of condition because it creates the demand for vintage images to be buffed away which is a great loss.