Before we start whole label witch-hunt, i'd clarify that drifty is a relative term, and I'm sure that any fresh new 3458A are well within it's specs unless faulty. It's just a common knowledge that any reference will take quite a while to get at best stability. Sometimes it's month, and with highest stability instruments that best stability plateau might take years.
Sure you know this already. Big part of metrology is statistics based and finding way to predict drift rates well, be it 0.001 ppm value or 1% one, is one of the important tasks, which take years
. And I'm also sure 34470A's are fine instruments, but they are nowhere to be proven like the 3458A is. Also you know better, that 34470A, even if you took it right from the calibration station at KS within 24 hours cannot be used to verify 3458A DCV specification due to 1:1 TUR (24hr 34470 vs 1 year 3458A). So one would need bank 732A/Bs with known history to be able prove or disapprove drift statement
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For newcomers to this thread: label only means when unit was last time serviced/made. There are many HP meters with KS labels, and perhaps some KS meters with HP label. It does not mean much, separate from the unit history/use case.