I see this secretarial version on this pose pop up often enough on sites like eBay or ioffer to warrant a post.

The secretarial:

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Here's one which sold on eBay with the original mailing envelope for 100 bucks. A mailing label with an autograph from someone who isn't a confirmed mail signer is always a big red flag to me.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&_trksid=p4340.l25...

Another one with an $89.99 starting price:

http://www.ebay.com/itm/Elizabeth-Taylor-auto-autograph-photograph-...

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This isn't the only secretarial autograph version, nor the only picture you'll find it on. The first thing I look at when deciding authenticity is the inscription if there is one because her cursive didn't change radically throughout her life. Here are a couple authentic examples of her signature: