As a Weights & Measures Inspector in my non-folkie life, I deal with calibrated weights every day. Every certified weight in the world is referenced to that single kilo weight kept in Paris. It has never been touched by humans and is kept in a vault underground. I wouldn't worry too much about 50 micrograms though. That comes to just a bit over 1/10,000th of a pound. Most working standards are not even close to that accurate.

Mike