Darklands Puzzle #20

A strong, iron door blocks the path. An inscription reads:
ALL FACES LIE OR ALL FACES SPEAK TRUE.
Six brass faces suddenly speak.
One: "The gold key opens the door."
Two: "The silver key opens the door."
Three: "Faces One and Two are not both liars."
Four: "Face One is a liar. Face Two speaks truth."
Five: "Faces Three and Four either both speak truth or both speak lies."
Six: "Face Five is a liar. Dwarfs are tall and willowy."
Three metal keys hang nearby. You ponder, then use...
...the gold key.
...the silver key.
...the copper key.

(Source: Darklands cluebook by MicroProse Software)

This one was badly worded. I thought the criterion was that either all faces are "knights" or all faces are "knaves". In fact, this is a standard "knights and knaves" puzzle.

Anyway, Six is obviously a liar because he described Dwarves as "tall and willowy". This means Five is not a liar, meaning that faces Three and Four either both speak truth or both speak lies. If they both speak lies, then, per Three's statement, Faces One and Two are both liars. But then face Four tells one truth and one lie about faces One and Two, which is not possible. Therefore faces Three and Four are both "knights". Accordingly, the silver key opens the door.