Corcom / Tyco IEC power entrance with RFI filter. 120 or 250 Volt, 50/60 Hz. P/N 3EZM4S. Built in on/off power switch (DPST), fuse, and four voltage range selector.
From manufactures data: "ZM Models – Premium RFI filter designed to bring most
digital equipment (including switching power supplies) into
compliance with EN55022, Level B (as well as FCC Part
15J, Class B) conducted emissions limits."