I'm very new to consider uncertainties in my measurements according to the GUM. And I don't disagree with your points, but as I understood so far it is possible to use an instrument better than it's specifactions. In the Fluke Calibration Books (Rev. 2) they mentioned to use the floor specifactions for transfers (equal value). Another described way is to make some measurements to characterize the instrument.
For example: I made 1000 measurements (10 NPLC everything above is just an average from the meter) with my 34401A (also 7,5 digits) on a 10k VHP101. This measurement results in a std. deviation (of sample) of 0.3 ppm. The standard error is even much lower after you apply statistics.
I would expect it is a traceable measurement if you make measurements at least over the duration of such a transfer. This should reflect short-term zero-drift, gain-drift and noise. Noise can always be reduced by averaging. But this will not reduce the drift. Therefore, one should be careful with the amount of averaging.
Another point is the linearity. But this error is very small due to the fact of the similiar values.
Edit:
I would calculate the uncertainty this way:
- AD linearity: 2ppm of value + 1ppm of range (Dr. Frank and others (inlcuding myself) already showed, that the INL of the 34401 is much better)
- To be very pessimistic apply the standard deviation of the whole measurement instead of the standard error and multiply by 2 for 95% confidence level
- Rs = 95% uncertainty of the standard
uncertainty = sqrt( 3ppm² + 0.6ppm² + 0.6ppm² + Rs²)
If we assume an uncertainty of 1ppm for the Standard we end up with 3.3ppm. The uncertainty will be dominated by the INL.
Did I miss a big thing?
Edit2:
So if you had a more accurate 6.5 digits with low uncertainty and high confidence vs. 7.5 digits with (much) higher uncertainty and (much) lower confidence...which one is the better result?
I think that isn't the question here. To enable an extra digit on the instrument will not harm the 6.5 digit specifications.