Thanks chrisyen, rockdaman, this is one of my top 5 favourites in my small collection.
According to GS spec sheet, SD is guaranteed to be +1 sec/day or +15 sec/month but in reality it is much more accurate, more like < +2 sec/week..perhaps the +/- cancels out over the week.
I have 3 SDs and they vary from +1 sec/week to < +2 sec/week, all single barrel but with 72 hr PR