The Apple 1 was, as its name suggests, the original Apple computer designed by Steve Wozniak. Unlike its successor the Apple II, it didn't come with a case but it was supplied assembled rather than in kit form like the Altair and other home computers of the time.
The significance of the particular example auctioned by Christie's was that it included a number of associated items including the all-important (to collectors, at least) cardboard box, plus the manual, original invoice, and a typed letter signed Steven Jobs.
The inclusion of these and other items helps explain the expectation of a high price, despite the fact that the computer wasn't in original condition. According to Christie's, the device included "a few slightly later additions" and some "later soldering".
Scarcity is also a consideration. The Apple-1 offered for sale was number 87 of a production run of perhaps 200, of which as few as 50 are thought to survive.
What price was achieved for the Apple-1? Find out on page 2.
In the event, the Apple-1 sold for £133,250 ($US213,600), including the buyer's premium. This was just inside the middle third of the estimated range.
But the highest price achieved on the day went to Isidore of Seville's Etymologiae - the first half of a 7th century encyclopedia - sold for £421,250 ($US675,264), considerably higher than the expected £100,000 - £150,000 ($US160,300 - $US240,450).
It appears that a number of lots failed to reach their reserve prices, including the Turing offprints. Efforts were being made to secure them for the Bletchley Park, where Turing worked during WW2.
A full list of the sale results can be found here.