There's a little bracket that takes the place of an axle spacer - when you remove the wheel, the computer goes with it! And there's a little pulley that attaches to the hub flange and/or spokes, and a rubber O-ring connecting the pulley to the computer.

And you are correct - the top number is the resettable trip meter while the lower number is the permanent odometer.

It's my favorite "computer" and I don't bother with the electronic ones. I see people staring at their computers while dripping sweat and trying not to vomit. That seems like an odd recreation to me. A record of mileage is kind of a neat (albeit completely unnecessary) thing to have, but a multifunction desktop-on-wheels telling me what to do doesn't make any sense at all to me. C'est la vie.

<edit> Most of the time there's one or two available on Ebay. http://www.ebay.com/itm/SACHS-HURET-...item4ab5fefb4f