Below are two version of the 1907 Imperial Exposure Reckoner. The top one is the earlier, evidenced by the lower speed range of film, running from 6 to 400 H&D (Hurter & Driffield) and the exposure time range being marked in seconds, minutes and also hours !

The second version, shown below the first, has a film speed range 5 to 600 H&D. The exposure time is only marked as seconds and minutes. There is also additional information and the Reckoner has an overall 'cleaner' (more modern looking) layout.