Hello,
I recently purchased an 04 LS430 and I’m reading the owners manual... I have 2 smart keys, 1 sub key, and 1 flat key. I don’t have the metal key plate. It’s #3 pictured below.

 

The key plate has the code numbers that enable the dealer to recreate the keys. As long as you have a master key (one of the two that come with the car), you don't need it. That said, the number is good to have. The parts department of your Lexus dealer can retrieve the code for you. They may require lot's of proof of ownership and for the car to be registered in your name at Lexus corporate. This means you have to send in the little card that is in the owners manual notifying Lexus about the change of ownership.

 

Its massively important. It has a code you need to enter into the radio every 12 months or 6,000 miles, whichever comes first, or the car immobilized via the ECU. It's an old school security feature they put in due to the high number of luxury cars being stolen a d exported in the early 2000s.


I'm joking, never heard of this thing before.... I'm guessing it helps when you need a new key cut maybe, so they know what design to use?

 

Bah, you ruined my joke by typing faster than me!