Well, I see that misdescribed HP 428B DC current meter on ebay didn't shift. This one:
Looks very tidy for its age. I was tempted myself but decided in the end that I didn't want the hassle of adding something with valves in it to a valve-free household.
I took the trouble to grab the manual for it and the principle of operation is, as is often the case with older simpler gear, quite interesting. None of your hall effect sensors, oh no, uses the DC magnetic flux to alter the operating point of a transformer being used as the sensor head, injects an AC signal into the transformer and then nulls that with another signal, the feedback signal that drives the level of the nulling signal is proportional to the DC flux and that is what gets indicated. Very neat, and only 3 dual triodes, 3 pentodes, 4 vacuum diodes as a bridge/synchronous detector, and three transistors in the whole thing (There's a couple of silicon diodes too, but they're just used to derive a bias voltage, they hardly count).
Well, that was the first thing of any real interest that I've seen on eBay in weeks. Everything else has been either "got one", "don't need that", or "You're asking
what for that!?".
Oh well, back to searching dolorously and fruitlessly for something interesting to buy, or so cheap that I can't
not buy it.