Drum brake that fits on a Brompton fork?
70mm is the diameter of the brake drum. There is model XL-FD which is labeled as "90mm Front Drum Brake Hub" that has equal width but larger drum. The respective pitch circle diameters are 89.9mm and 109mm, which places the spoke holes 9.5mm away from the drum.

So, the OLD is 100mm, which is standard for most bikes but unfortunately does not fit Brompton.