I bought a 76 and a 78 new. In 76 it was a suspension option on a D100 and the truck came through with no emission controls. The 78 was badged as a D150 and it was the same package.
I tell you it was a great vehicle to buy in the mid 70's, the cars were full of emissions jeejaws and hoses and none of them did any good anyway. Pop the hood on a 150 and it was clean as a whistle under there.

Sheldon