Ralph,
My records indicate that the current Series of MGM Roulette chips are known with the following Letters.
A-B-C-D-E-F-G-H-(no I)-J-K-L-M-N-O-P-(no Q)-R-S-T-U-(no V)-W-(no X)-Y-(no Z).

The U chips are used at a private wheel(used only for special guests) in the High Roller, Baccarat Pit.

The W chips are used at a table in the Baccarat Pit that is open to the public $25.00 minimum chip value and at least $100 per spin minimum.

I'm not exactly sure where the Y table is located, but I'm pretty sure it's located somewhere on the main Casino floor.

There are 21 known letters in this current Series of MGM Roulette chips.

Casinos often skip or don't use certain Lettere or Numbers for various reasons.

I hope this helps.

Take care,
"Mr. Roulette"