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Although the case isn’t marked with the name of the alloy, i think it is a patented alloy called STAYBRITE which was extensively used in the fabrication of many top tier Swiss watch case like Omega and Rolex, and possibly others.
Very few nickel plated cases for pocket watches. It's either 'métal blanc' i.e. nickel based alloy, or a staybrite produced before Omega advertised them as such inside.
I presume it is a ref CK395, which is contemporary to the first use of Staybrite by Omega around 1934.