To rewind the stator winding of an automotive alternator, insulation materials are crucial. A 'U' shaped thin sheet insulation should be placed in each stator slot, and rectangular coils can be wound into these slots. Enamelled copper wire is typically used, along with insulation paper or cloth, such as electrical insulation fiber paper or Nomex. The cloth material seen in the photos is likely a type of insulation cloth designed to separate the copper winding from the steel body. Proper insulation is essential for the longevity and performance of the alternator.