Both black keys do the same job, from memory the grey doesn’t open the glove box.
My first SC430 (2003 model) only had one key, so following advise from other Lexus forums, I purchased a used 3 button key from eBay, also purchased a new blank key.
Took car and keys to automotive locksmith, he cut the key, reprogrammed the guts from the used key, carried out the procedure with a mix of door open then shut, ignition on/off, foot on brake etc….was a 8-10 part process and all done for £75.00…..note that’s 2016 prices 👍🏻
What goes against you now is finding a used key and a auto locksmith that’s familiar with close too 20-year-old technology ☹️