What is the power plug in the photo called?
Apparently, there are HP server power supplies that have a non-standard connection.
They are found on ParallelMiner as KT-1200-200V-1 for example.


  We use a custom 12inch power cord to bypass the proprietary AC plug


Behold the custom cable:



It's just a cable with quick-connects.



Apparently this is the 277VAC version of the power supply:  

HP 1200W Common Slot 277VAC Hot Plug Power Supply Kit (717359-B21).  

The plug and socket are called LS-25 and LS-26. (an adapter is sold by stuartconnections)
HP also sells adapter TK801A and TK802A.



(source)