Once you have the masspiece set you will need to send it off for certification. I know my set at least down to the nanogram resolution, and as the massmeter used to reference it is actually verified by a masspice, and this massmeter is also used to self calibrate itself every 6 months using the local SI kilogram reference unit, I am pretty sure of them being correct, and traceable to the standard kilogram to a good few decimal points, seeing as the error between the standard kilogram, and the new SI defined kilogram, is in the order of a few hundred picograms, but is now reproducible anywhere, without needing to take a trip to the landlocked separate country in Paris that houses the SI standard.
Knowing I have a 1.0000g masspiece as reference, and a balance that actually has the ability to see the 1.000g mass, is the start of that rabbithole. And yes, I have seen that Platinum iridium alloy mass, in it's glass case in the safe, within a safe within a safe, cut into a hill in a vault by me, a few times.