if a regular line entry has a fully defined date range (e.g., 1977-1981, Mar-01 - Dec-03, etc), then it is retired. If the end date is open (e.g., May-01 -, etc.), it is not known to be retired. Note that specials may have a date, but no range.
If the date is '?', it means we'd like your help in filling in that information! :-)