Phyllis,
Good question.

The chip you show, I think is more corectly described as a Bud Jones "Autosort" style Roulette chip. It is similar to a plain mold chip but the center inlay is recessed. This style of chips was made to be used with the various automatic Roulette chip sorting machines that are used in many casinos.

I think TCR (Doc Myers are you listening)and Jimmy C. also should refer to these chips as "AUTOSORT" not Plain mold because of the recessed centers.The Roulette chips from MGM, Mirage,Monte Carlo, Ballys, Luxor, etc. are all examples of this type of chip.

I just looked in TCR and all the MGM Roulette chips are listed as "Plain" Mold. I think the First and Second Series of chips should be refered to as "Autosort" and the current Series should be refered to as "24 Lines" which are a further modification of the autosort chips that have 24 lines scribed in the outer rim of the chip with a recessed center inlay.

Technically I think of Plain mold chips as having a "flat" or "smooth" surface with no center recess or indentation.

Shown below for comparison are left to right:
1. Carson Nugget - Plain Mold - no center recess
2. MGM - Autosort - Series 1 - recessed center
3. MGM - Autosort - Series 2 - recessed center
4. MGM - 24 Lines - Series 3

Just my thoughts,
"Mr. Roulette"