There were two varieties. The type in my pics above was the navigators type, you can see the remains of two of the four feet on the top pic, where it lay on the chart table. This was because most use was done on the circular window where wind speeds and direction were fiddled. One then turned the contraption over to use the circular slide rule on the bottom to calculate times and distances etc., as well as determining true airspeeds and heights with the little scales in the windows.
The other type had that section on the hinged lid and with leg straps, for use by a pilot who had it strapped to his knee. These were in the main used by the PR pilots, I cannot begin to comprehend how these men managed 4 hour plus sorties over a very hostile continent, and maintained accurate navigation in the very confined space of a Spitfire cockpit.
There was a later variation that came with the jet age. In this the cloth band was replaced with a plastic slider which could be reversed to give a greater range of speeds. This slid through another plastic gimmick with a 360 rotating window to do the same job.