As Sheldon said it is really 2 different casinos. The original "MGM Grand" where Bally's is now are listed in this group:

http://www.themogh.org/cg_chip2.php?id=NVLVMG

The current MGM Grand Theme hotel is the big green one you see today on the strip (site of the old Marina, then MGM Marina) Chips shown here:

http://www.themogh.org/cg_chip2.php?id=NVLVMT

Hope that helps!