Is "bronze and silumin" mix actually possible in a ball cock?
Well, I cannot find a conclusive answer but I have a very educated guess.

Bronze is an alloy of mainly Copper(Cr) and an additive Tin(Sn). Silumin is an 
Aluminum(Al) alloy with 12% Silicon(Si). It is highly resistive to corrosion under wet environments.  

During the alpha phase of metalic cooling, rapid crystallization occurs that is visable under a microscope at 5 µm. I will spare this answer from the nitty-gritty but with the addition of Sn to a bronze alloy, the hardness increases by 55 on the Brinell test. 

Ergo, if you added Silumin to Bronze, there by adding Si to bronze, you make is harder. So it is plausable to say this mixture is used for this application, especially since we are dealing with plumbing.

Spoke with my college chemistry prof. who agrees that this is an entierly possible mixture of transition metals.

(will post pictures once I find them)