My employer recently purchased a cabinet full of plug and ring gages for various threads at auction. What I am finding is a lot of them simply have a N designation after the thread size. I have found this to point to them being quiet old. What I am curious about is if the are still usable for a UN thread? What I gathered from the ansi standard that I read about is it was updated in 1949 for the standard to be changed from n to un. It has been updated since as well. The only difference I have found is that the pitch diameter was changed ever so slightly at around .0001"-.0002". What I haven't been able to deduce is if the gages are usable or obsolete?