Definitely not radioactive decay, BillD. Longest half life of an unstable isotope of Iridium is 75 days. That means that any noticeable mass loss from this source (to a resolution 50ug in 1kg) would have happened within 5 years of the kilogram's manufacture. There would have had to have been a lot of it, too, as it doesn't just disappear, but converts to another element, which I don't know off hand, but the products are likely to be similar in mass.

In any case, loss of 50ug in 118 years, e=mc^2, that's about 1.2W, which would certainly have been noticed.

I assume those wedded to traditional systems have memorised all the various fothers, and know the difference between a statute mile and a Swedish mile, not to mention pounds avoirdupoids and pounds troy?