I searched up a manual that wasn't on Butkus and found two different versions of the manual, implying two versions of the Automatic camera. The 1/2 second shutter speed may be the biggest difference, or perhaps the upgrade model had a more sensitive selenium cell. I do note that the meter calculator in the photo above has 2, even though the shutter speed dial doesn't...
Looking at http://www.submin.com/16mm/collection/mamiya/index.htm my tentative conclusion is that the earlier models had the extra 1/2 shutter speed, as far as I can tell it is only the last two cameras on the page that don't have it and they have the highest serial numbers, 2457032 and 2495996. My Automatic has a serial number in between those, 2468710.
Well, that would explain the ones without 2 on the shutter speed dial still having it on the exposure guidelines. Makes some sense, anyway; 1/2 second is easy to do with a cable release, so having the camera time it is redundant.