The wheel bases changed from short to long in 2004. This change happened because the hinge was improved to a stronger joining method known as lug brazing where the tube is brazed inside the hinge plate. This meant that the frame also became slightly stiffer. The previous hinges were butted, meaning the tube was brazed up against the hinge.
The short wheel base Brompton is 1015mm.
The long wheel base Brompton is 1045mm.
For further reading on frames, read this article here.