SPT-1 vs. SPT-2: SPT-1 and SPT-2 is a measure of the insulation thickness of traditional 18 AWG cords. SPT-1 is the type most commonly used by commercial Christmas lights installers. SPT-2 is thicker and tailored to harsh weather conditions. Be sure to order Gilbert plugs that match your projects.
If you are purchasing plugs for existing cords, check for the SPT rating printed on the insulation. If there is no specification stamped on the insulation, there is a good chance that the cord is SPT-1. Check with the manufacturer or the cord's packaging for confirmation.