A Michael Myers mask that seems to be from the film Halloween 4: The Return of Michael Myers but used during the production of Halloween 6: The Curse of Michael Myers was discovered recently.
Member MrDillweedius shared photos with us of a Michael Myers mask that was given to his family after shooting of Halloween 6 wrapped up back in 1995. MrDillweedius has stated that the makers of Halloween 6 gave them the mask and that he is the brother of the actor who played Danny Strode in that film.
This particular Michael Myers mask has lost its hair and it was kept stored in the basement for many years. The mask does look like a Halloween 4 mask and it also may be the mask that the mysterious Background Michael Myers is wearing during Halloween 6 in a quick scene when Debra Strode is on the phone in her house.
Photos of Halloween 4 / Halloween 6 Michael Myers mask courtesy of MrDillweedius: