I appreciate the sanity check on prices; I had another look last night and got much the same answers. I've already replaced the dodgy caps in the unit as well as substituted for a missing unobtainium pot, so it's no longer "vintage" in any way. In my view, spending nearly as much for a cord as I have invested in the unit is crossing the line and at that point I think upgrading the socket is the right way to go. It'll be safer in the long run as well, I believe.
I like Smith's approach, though I'm sure the metal socket is elusive as well. I'm going to a couple of hamfests over the next few weeks, and if I can't find a cord in a "make offer" bin I think I'll just go with the updated IEC connection. Thanks to all for the input.
The metal socket is easy to find & cheaper than the power cords, search ebay for PH163 and you should find some sockets.
Yes the cords for 1960's HP equipment have reversed polarity as has already been mentioned, new replacement ones should be labelled to indicate this (see attached Volex 17952 PDF from 2007), Volex discontinued them a few years ago, but there seems to be no shortage of new or NOS cords on ebay under various names including Belden, Volex, Alpha, etc.
Sphere have some new Alpha 543 ones here (scroll to the bottom of the page);
http://www.sphere.bc.ca/test/wallwarts.htmlI bought a couple from them a few years ago, they always seem to have the comment about them being hard to find or last ones on their listing, but always seem to find more.
Also note that IEC sockets can also fail after many years, I've changed a few with bad earth/ground connections due to the crap riveted type construction of the pins. And I expect after 40+ years the pins on IEC connectors will probably get tarnished too, especially if the equipment is stored badly in damp sheds/garages or attics/lofts like a lot of vintage stuff seems to be, not that that can't be easily dealt with a bit of cleaning.
By the way if you end up buying more old HP equipment you will probably end up with plenty of the old cords, just like I have.
David