Ken is absolutely correct, no 6.9's were produced in 81, last cars were made in 1980. In fact I doubt the 4 cars sold in the US in 81 were actual 80 model year since for 1980 all vehicles sold in the US had to have an O2 sensor to meet emission standards and as far as I know no 6.9 was ever produced with K-Lambda fuel injection.
Tks,
Joe