One of the Haunted Mansion envelopes used by the management to send correspondences to actors and employees.  It only cost 15 cents to mail a letter in 1981.  Wow!  A special thank you to Barragonis Wreap and Janice Dixon for providing us with this copy of the envelope.

This is the back of the envelope.  If you look close, you can see the envelope seam along the top.