Nice! I was lucky, the Toyota dealer here had 3 blanks from the 80's era. So I have a nice new key that matches the period!
The plate was a small rectangular plate that was punched to go on a key ring and was about 5mm wide and maybe 2cm long. It has a 4 (maybe 5?) digit code engraved on it that can be used to make a new key for your car. The idea is that you kept this somewhere safe and then a key could be made at a Toyota dealership if you lost all of your normal ones.
The down side is that this number can be looked up by a Toyota dealer by using your VIN. So it is possible that someone could grab your VIN and get a key made for your car that way. (Most dealers are good about requesting a title first, but I know people who cover the VIN up on their dash out of concern.)
The other funny thing is that the same lock cylinders show up on multiple cars. I think a Camry and Corolla have the same key set as my car