OK, I've ordered a Chinese component tester.
I've also taken a larger handful out of the bag. They're all marked 6-5910, some are marked ECC343 instead of ECC074. There's no marking on the mounting flanges which would indicate they'd ever been used.
The case does not conduct to any of the pins (or at least at multimeter voltages). They conduct in both directions from 1-3, but 2 doesn't show any conductivity to 1,3 or case in either polarity.
I'm very intrigued as to who made them, there's no manufacturer's logo on any of them.
Thanks for the suggestions thus far.