I'm no expert on this subject and don't know the answer but I do know that many Cambodian's feel that their art, Muay Boran, is where Muay Thai came from. I live in Thailand and I've been to Cambodia many, many times. I've watched a few fights and the difference between Muay Thai and Muay Boran is that Muay Thai is more refined, not so wild and Muay Boran uses more elbows.
I've heard it referred to as "Khmer boxing" for Cambodia, and also for Laos. I know they argue back-and-forth about who developed it first, Cambodians or Thais.