Interesting.
The listing shows the model year 1981, and the listed condition is: "Like New".
[
attachment=10050]
If the ad is accurate the engine has been replaced, I believe the first 6-cylinder Cummins to leave the Newell factory was an ISX much later than 1981.
That is either a 1980 or 1981. Based on the VIN I show for that vehicle it was built prior to January 1, 1980 when the new federally mandated VIN format was required. It has the Cummins 555 which is an 8 cylinder engine even though it is listed as a 6. The VIN indicates it is a 37' rather than a 40'.