Evidently there's no answer to the 920/920 super difference question, probably just one of those marketing type deals. Some think it's maybe the modified bottom fed lower transfer thing a la 930 but there's conflicting reports there.
As to year, I've found that confusing too. Acres lists it as '82-'86 and the 930 starting in '86, though I've seen at least one '88 920 tag and no evidence that the models were produced concurrently. The serial numbers are funny in that there are the short numbered ones with the year and long numbered ones without but there are the dated ones seemingly randomly interspersed throughout the whole production run.
So it doesn't seem easy to figure even a range of years on the long serial # ones other than jonsered did seem to be consistent in keeping the numbers sequential.