'Ur', as in the phrase 'Ur example' or 'The Guttenberg Bible is the ur-book'.
[Edit 03/04/2015]
"Forming words with the sense of ‘proto-, primitive, original’."
Source; Wictionary http://en.wiktionary.org/wiki/ur-
"The Ur Example is the oldest known example of any given trope. Ur-' is just a German prefix meaning 'proto-, primitive, or original.'"
Source; TV Tropes http://tvtropes.org/pmwiki/pmwiki.php/Main/UrExample
An "ur-book" would be (one of) the oldest known example(s). In that context, the Guttenberg Bible would not be an 'ur-book' but it would be the ur-example of the printed book.
The ENIGMA machine could be thought of as the ur-computer. It almost certainly wasn't the first functional computer but it is the first example that is generally known. Charles Babbage's analytical engine could also be thought of in this way, as it was conceived at an earlier time, but it was entirely theoretical and no contemporary examples were ever constructed.