I can't prove the dating either way, but I too would bet on a rendering for the original hotel, not the expansion many years later. I stayed at the Rio within the first year after opening. Very colorful and a nice "flow" for casinos of its time.

My reasoning: These are hand drawn on matte board, which is more consistent with the original tower. By the early 2000's, I think architects were using computer-aided and -generated renderings, which would be printed on large pieces of paper, not art board with water colors, etc. I also agree that the people look to be from an earlier era, not the early 2000's. Under magnification, do you see paint and brush strokes?

If you can do some research on the Marnell logos over the years, and their addresses and phone numbers, you may be able to narrow the dating. I think Marnell did move around and update logos a time or two.

Lastly, take a look at renderings for other casino projects of the early 2000's, such as Wynn. They were more advanced, more detailed, more computer-aided. In contrast to the hand drawn pieces you show.

Interesting find.