This is a (poor) closeup of the signature by Charlotte Stewart ("Mary X"). Ms. Stewart also went on to play "Mrs. Briggs" in the Twin Peaks TV series. This was signed at the 2000 Lynch Festival at the Seattle Art Museum. (See below)

Just below her signature you can see three dots and a line - a crude face. Jack Nance (Mr. Eraserhead himself) would use this icon as his signature. Ms. Stewart placed this isignia on the poster in memory of Nance who had died two years earlier.