The case is authentic. The caseback says clearly that it is a chromed metal case (métal chromé) with a steel caseback (fond acier), a fact confirmed by the first digit "3" of the UG case reference engraved on the back.
When a case is chromed, it is by definition not steel. There is no need to chrome stainless steel cases - they are already rust resistant and can be polished to the desired finish.