Basically, the internal ports/passages get more "aerodynamic" inside. Rounding the corners going into those pinhole orifices, internally.

In later years, they tried to do this(on brand new nozzles, for EPA reasons)) by "extrude hone" method;  later, 3406C model. This didn't work so good.

I've been told that it is actually the very small debris/particles in the diesel fuel, over time, is what causes the wear internally. Remember, most people only run 10 micron fuel filters, back then, too.