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.
Agree that the case is authentic. But I'd caution you to never polish a chrome watch case (Universal or otherwise). The chrome layer that covers the unelegant base metal core is quite thin. In my experience, attempts to polish the chrome layer have been invariably disappointing with results that look worse than when I started.