Mac,
The third letter of Taurus serial numbers denotes the month of manufacture, and they're A-L. Your NUG serial numbered pistol was manufactured in in July, 2001, so it probably sat around in the distributor's or dealer's inventory until you bought it.
Since serial numbers are an issue the authorities are really uptight about, I wouldn't be surprised if Taurus doesn't do what Kel-Tec does. When they have to replace a frame (which has the serial number on it), they mark the new frame with the old serial number.
I think when you get your PT-145 back, it will have the "fixed" frame with your old serial number.
I recommend reading the thread StratfordHoldings posted the link to.