My guess is that they were more likely than not used at the same place, whatever that place actually was. It’s rare for a gambling establishment to order just one color of a chip.

I also wonder if this UFC chip submitted to the ChipGuide by Ed Hertel is related to the yellow and brown ones. Maybe at a later date the owners of the yellow & brown chips needed a 50 cent chip, but no longer had the hot stamp, and had a new one made. The mold is not very common, a galloping horse is an unusual design.

(courtesy of the ChipGuide)

Did your friend find the Yellow horse hot stamp & Mecca chips in Newport, or in mid-Ohio? If mid-Ohio, the origin of those chips might be closer to there.