Verified by manufacturer, the HL 1620 S is a basic heat gun model. No reducer or reduction nozzle will work on the 1620 S, or even the 1820 S. The nozzle reducer is only for heating tools with electronics built into the heating tool, such as 1920E.
With the reducer nozzle the heat gun will put more heat back into the heating tool because the end of the nozzle is so narrow and the extra heat has to go somewhere. The extra electronics on the 1920E will regulate the heating element and the motor in the heating tool. So, if the heating tool starts to get too hot internally, the electronics will turn the fan up and the element down.
More information about Heat Guns:
Heat Guns, Torches, Acessories - Design Tools and Resources