Wikipedia has an article on the International Prototype of the Kilogram, which also includes a graph of the relative weight of its copies. Because there is (were) no independent unit to measure weight, every copy's weight were measured against the International Prototype.
It seem that most of the copies gained weight relative to the kilo over time, but it seems more likely that it lost weight¹. Considering the chart below, how would one make an estimate of the weight of the International Prototype in 1989 relative to its weight in 1889? Is it as simple as assuming a Wiener process and taking the average, or are there other things to consider?
(The label numbers denote the other prototype copies, not weight.)
1: But it's also possible that it gained weight, just less so, or that it were in fact stable.