"The documents also included the only known examples of Banbury sheets, a technique devised by the mathematician Alan Turing to accelerate the process of decrypting Nazi messages. No other examples have ever been found."
The structure and use of Banbury Sheets appears to have been well known prior to this discovery? TBH I'd say a specific piece of paper someone drew or wrote on isn't important if we know the content of that paper already; but I know many would disagree.
Well, sure. Original, handwritten documents are almost always considered more valuable/important than nth-generation copies.
E=MC^2 is going to be a lot more valuable in Einstein's handwriting than it is in this post. An original, signed Declaration of Independence or Magna Carta is a national treasure, but a printed copy won't be worth more than the cost of printing.
http://www.fourmilab.ch/etexts/einstein/E_mc2/www/ says the original, translated from German, was the text "If a body gives off the energy L in the form of radiation, its mass diminishes by L/c²", that is, m = L / c². L was used for energy at that time, instead of E.
There is no square root. If you include the momentum term it's E²/c² - p² = m²c², which solving for E gives a square root, leading to http://en.wikipedia.org/wiki/Dirac_equation .
Was the third paragraph too hard to reach?