MGM Grand dice:
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There are several possible reasons why the letters appear to be different:
1. The dice are from different series;
2. The two dice were stored at different places for a while. If you have one die exposed to the light and another in a drawer, the logo of the first die will fade over time.
3. The dice are reproductions.

LV Hilton dice:
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I can't find the LV Hilton dice on your picture in the Laudeman and the Olson/Johnson book nor do I have a similar pair in my own collection. Here is a theory and I can't proof it: The dice might be reproductions. I have a picture attached with a common pair from the LV Hilton and you can see that the location logo on the one spot matches with your pair. Now, the AC Hilton used the 2nd pair on the picture and you can see that the logo on the 2 spot is the same as on your "Las Vegas" pair.