I have 2 stained glass 'tiffany style' lamps one is marked SPECTRUM on the inside and both are marked SKC where the lamp shade sits on the lamp What do those markings mean?

The 'tiffany style' lamp marked SPECTRUM means that it sheds a full spectrum of light. The SKC marking just stands for the manufacturing company where the lamps were made and shipped from. It probably has to do with identifying them as a specific type of 'tiffany' lamp.