Plenty of letters in the alphabet.

Worked for Star Trek.

There will be fill ins on numbers they never used-

940, 990, 984, etc. There's a lot of numbers they haven't used in the 900's. For instance, the new 918- they had never really used 918 before (there was the flat-8 914 car that was built for Ferry, but 918 was never officially used)

Then they will add letters. Like with the 9R6.

99X'd have to be a classic-rock themed car. Shag carpet and the works. Kind of up the ladder from the 97Y easy listening sedan.

But they HAVE used those numbers...

940 was the Indycar.

990 was another race car, etc.

Porsche uses numbers sequentially for all sorts of projects; 356 was just project #356, #901 was the 911 (see above).

They just haven't all been cars.

Some of the numbers above have also been used by SAAB...