I don't know when it was made, but it's worth no more than about $250.00, if it's in excellent (new) condition.
The "T" in its serial number leads me to ask if it's the "Sidekick Trapper Edition." If it is, there's a special stamping on the barrel somewhere that reads "NTA." That might raise it to $275.00.
The Sidekick was made from 1956 to 1985, discontinued, then made again in 1996, and then discontinued again.