The 1984 GT350 20th Anniversary model was produced in March-June 1984 to celebrate the Mustang's 20th birthday (5,261 were built).
All 20th anniversary Mustangs were white with rocker panel tape treatment and GT350 lettering. The interiors were reded with articulated front seats and two 20th anniversary dash panel badges. A medallion engraved with the words "Limited Edition", a serial number, and owner's name was sent to the buyer about 4 months after purchase.
You can go here to decode your VIN:
http://www.mustanggt.org/main.htm