You're really testing my memory from when I was a Nuclear Machinst's Mate in the Navy (circa 1981), but if I remember correctly they go as follows:
USS= United States Standard (developed by the Navy)
SAE= Society of Automotive Engineers
but then you have
UNC = Unified National Course Thread
UNF = Unified National Fine Thread
I thought we were on this Unified Screw Thread Standard
Except for ISO = International Standards Organization (Metric)
Best that I remember, they only use the USS and SAE standards for washers anymore, then it's just large diameter vs. small diameter.
I am remembering all of this 25 years later, so it may not be all there.