IEC 309 or BS4343 Commando connectors are produced in many variants, designed so that a plug of one type can only be inserted into another socket of the same type. Different current ratings (eg... 16 Amp, 32 Amp and 63 Amp) are distinguished by the different colour and diameter of the circular housing.
IEC 320 C19 and C20 connectors are oblong with three pins or sockets and are rated at 16 Amps. Generally they are used for high power servers, UPS's, PDU's and some HP / Cisco equipment.
They are not to be confused with the smaller C13 and C14 (Kettle Connectors) which are only rated at 10 Amps.