What is this AC power connector for a Hughes TWTA?
Thanks to Glen's hint, I managed to track the cable type down - it is indeed a Hewlett Packard receptacle from the 1970s. It is either known as a 163 or PH163 connector, and was one of the more popular of the predecessors of the IEC C13/C14 - used from 1959 until the late 1970s.

If you need a cable of this kind you can probably find one under the manufacturer name of Belden or Volex (model 17952).

You can find some more info on obsolete connectors from this page (unfortunately only available via Google caching ATM...save it if you need it):

https://webcache.googleusercontent.com/search?q=cache:AbLwc6-pTpMJ:https://www.cs.ubc.ca/~hilpert/e/powerConn/index.html+&amp;cd=1&amp;hl=en&amp;ct=clnk&amp;gl=us