I am not dogging Walking Dead, but I was wondering, did you try calling a few vendors to see if someone was sitting on a Metallica Premium somewhere?
Don't give up on your dream pin. Its not that old and its not impossible that they are out there.
Just to name a few to try: (feel free to add to this list if anyone has more)

Game Exchange JJ - 1-303-931-3685
Little Shop of Games - 1-561-248-9628
The Pinball Store - Mike - 1-800-727-8363
K's Arcade - Keith - 1-877-775-5399
Tilt Amusments Trent - 740-803-2276 (Doesn't show sold out on their website).
Betson - Joe- 1-800-274-6326
Lanes and Games - Mike - 1-617-876-5533
Cointaker - Melissa or Chris - 1-570-847-9595

I didn't check the other vendors sites, but I happen to see Tilts real quick while looking for their phone number.