I finally found the puzzle solution
At pag. 9 of the book "Manuale dell'argento" di Giovanni Buzzi ( https://books.google.it/books?id=1SKVAw ... &q&f=false
)As for investment ingots, normally made in pure silver , it's easy to roughly check the authenticity: just compute the volume (width X thickness X height) in cubic centimeters and multiply by the specific weight that in this case is equal to 10,49. This rule does not apply in the case of ingots produced by 'Metalli Preziosi", a former market leader. The M.P. coined the investment ingots starting from a powder obtained in chemical way, not from laminated sheet. The powder was pressed and then sintered, a heating process near the melting point. The coined ingot in this way has a much lower density and to meet the weight it is necessary to increase the volume, usually the thickness. It's all perfectly legal , but you can not use the method described above and the potential buyer might be misled by being in front of an ingot with an increased volume.
You can not distinguish, just by looking, the difference between a coined ingot and this sintered one, but finally everything is explained.