@jolter I did end up going for nickel plating over the bronze valves.
The casings also turned out to be nickel (German silver) and could possibly have slightly different alloy composition than the valve replate, but I can't be shure.
So far, I have not experienced any problems with valve action or unusual wear on the valves.
I don't play it all the time, only in rotation with other vintage horns.
G.