Cameraeccentric has two scans of old Imagon documents: and

Your shutter most likely needs to be serviced, or at least test it to see what speeds you actually get at each setting. Yes, you use the same H-stops value as F-stop when determining exposure, but if your shutter is a stop or more off, you're going to have problems. If you're not familiar with compound shutters, you can easily break one if you try to cock the shutter in "B" or "T" mode ("T" might be labeled "Z" on your shutter). Here's a video of the Imagon 300 in box, and Eddie shows you how NOT to break your shutter:

I have a recent (post 1980 I guess) 250 Imagon in Copal shutter with all the disks and the 4-stop ND filter (more useful these days than the 2-stop green filter) and mostly use the 7.7/9.5 disk, at either setting. You'll get different results using the 5.8/7.7 disk at 7.7 than you will using the 7.7/9.5 at 7.7, so you'll want to experiment to see which one you like. As others have said, you're welcome to use the aperture either with or without the disks. I sometimes stop the aperture down w/o any disks to help focus, choose a disk, re-confirm focus, choose a disk setting, and then shoot.