Most SEs made before mid-1988 (the ones with specs listed on the rear bucket) have a 1986 copyright date--yet only one (the prototype I own) is confirmed to have been made that year.

Both the IIcx and SE came out in March of their respective years (1987 and 1989). Chances are the designs were finalized sometime in late November or December, then the factories were tooled to produce them en masse.

Sometimes, parts in computers will also have older dates if they are interchangeable. The Centris 650 I just put up for trade had a power supply dated December 1992 from its code (the Mac itself was made in February 1993).