Andreas,
Maybe I missed this from previous posts, so forgive me. Your data looks pretty detailed, but usually we're used to seeing the other critical & helpful section included somewhere data reports. I know that's not what you're doing here in a brief summary, and I'm not complaining at all. It might help interested readers later on though - so the following is just a suggestion for "best practices" for anybody reporting low ppm data:
When you publish low-ppm data, it would be very helpful if you can add an equipment list with cal dates, and standard calculation of your lab's overall measurement uncertainty (for absolute value, not relative) @ what confidence level? It's also handy to add a measurement uncertainty for each test and technique separately in a separate column, so one can see what you measured for an absolute value @ what error band size. For instance: DMM in one test, uncertainty 8ppm, and null meter + KVD + 732 on second test, uncertainty 4ppm, etc. Or whatever technique you use to estimate a voltage measure.
It's always interesting to know the error band of each measure, because it's all just an estimate at low ppm - every measure always includes some level of uncertainty and something less than 100% confidence. When you put that information in with your data measures, it makes it more meaningful when comparing experiments.
What do you use for Volt Reference in your lab? - do you have something other than LTZ-based Vref to measure LTZ's against? I see you are making comparisons to LTZ#2, but is that the only reference you have?