Difference Engine
I am sorry to report that the one from the Computer History Museum is, well, _from_ the Computer History Museum. Its owner has reclaimed it and moved it a ways north.
Some nit-picks:
The difference Engines are not computers. More like a Systolic Array (or a stack of adding machines) feeding a "typesetter" (which is not quite that either).
There are more than 2 Difference Engines in the world, unless you consider the two from the Science Museum to be the only ones that count. For example:
http://www.meccano.us/difference_engines/index.html
Which lists several, some made from Lego, some Mecanno, and at least one made (in 1859) of the sort of materials one would expect from a proper Victorian Computer.