Steve,

Yes definitely old school.  My memory is that before we really approached all parts with ESD safety in mind, early FET transistors were extremely sensitive to ESD events.  They had no built in protection on the inputs as parts do today.  Thus these metal rings shipped on the parts to prevent a differential voltage between leads.  I believe that the gate insulation is the area at risk.

If my memory is correct one lead is tied to the can to act as a shield when the part is installed in the board.

Drew