I think she's an Argentina "Twilight". No clue on the price though.
Argies like that aren't prototypes so much as they're unfinished or hair/paint tests. You can find almost any combination of symbol with either white or orange hair. They're really diverse! :)
It is believed that these ponies had their symbols added at a later date and the ponies, tails and accessories came from a toy factory when it shut down. Unfinished ponies are missing paint work, manes and tails. Some have no plug holes or tail hole. "I bought these with and without symbol about 10 years ago (2006) from an argentinian seller. The background story that was told to me is that when the factory shut down many employees took the unfinished ponies with them. And that could be a reason why there are a lot of blank ones." -Barnacle_lady