Hi - that's a very fuzzy picture...but from what I can make out, I suspect this is one of those vintage Bulgin connectors. Sorta like the 1950s version of modern-day IEC. IIRC - they come in a small and large size.
Here's a view of what those bulgin connectors look like:http://www.mullardmagic.co.uk/mullard/p ... tl121.aspx
As the link suggests, these are used on the famed ESL57 speakers and other vintage audio products. So to get these plugs - you will be competing with the audio crowd for a relatively scarce supply.
Suggestion: When I've encountered odd-ball connectors I generally try to swap them out with the IEC sockets if feasible. IECs can be had usually $1.50 each unless pulled off a dead PC, etc. (which cost time/trouble, but is doable). This weekend I will be doing just that for a Tek scope under restoration (broken 'plug' receptical on the back end). This also helps reign in the power cord varieties as well.