Good news/Irrelevant news:
First the irrelevant news. It turns out it was 4 years old, not 6. I confused it with another TT 420 I own that's been sitting in a box because it doesn't have that seperate 4-pin mobo connector.
Good news: Because it's 4 years old, it is covered by TT's 5-year warranty, so I should be getting a brand new 420w
PSU eventually. I plan on putting it in Box #3 which has been showing signs of an aging
PSU.