Drum brake that fits on a Brompton fork? I'd like to install a drum brake on a Brompton.

The internal distance between the drop-outs is 74mm.

I don't know how wide the Shimano Nexus Roller Brake are, but Sturmey-Archer's X-FD looks interesting.

However, I'm confused about its dimensions: The page says it's 70mm while the PDF says its OLD is 100mm.

Can someone confirm 70mm is actually the width of the drum brake vertically, and 100mm the distance between the two flanges, ie. it won't fit on a Brompton?

Thank you.
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.