In 1889 40 identical platinum-iridium kilogram weights were delivered to the International Bureau of Weights and Measures (BIPM). One was selected as the International Prototype Kilogram (IPK), four were kept as working copies and the rest were delivered to signatories of the Metre Convention. Another 40 were delivered in 1946, two of which were kept as working copies. On delivery each weight was calibrated against the IPK. Recalibrations were carried out in 1946, 1991 and 2014. The calibrations showed that the weights were not 100% stable relative to each other. This chart shows the variation of BIPM's working copies against the IPK.  Note - the working copy with serial number 8 was stamped with a "41" in error, hence it is referred to as 8(41).
Own work
Martinvl