Yup, you've got it right.
That's the standard front rail clamp. I can measure the one on my 90 in a bit if you want (got a euro military canvas on it)
The black plastic lever, as you rightly say, goes at the "bottom", kind of sits up inside the lip of the windscreen frame. Think it's an 8mm hole that you need up through the windscreen frame and up through the clamping "header bar". between the top of the windscreen frame and the underside of the header bar, the spring goes in.
Then on top, is the washer and nulock nut.
It should be set up so that the nut is loose enough to just un-clamp the header bar, and clamp it down again, but (here in the UK) the canvas is so rarely removed, i tighten up the nut with a spanner as well (doesn't have to be silly tight) just to give an extra bit of seal on the header bar.
------ Follow up post added June 15th, 2013 04:16 PM ------
This site will show you some pictures of what's done. They're a great company, not too far from me either, it's where I got all the bits for the latest project.