I just pulled out the only reference I have that has an AR-10 in it. The captions of two that clearly have wood handguards (and plastic buttstock) are described as experimental. They had different barrel lengths and built-in grenade launchers. However, I suspect that all production models were made with no wood at all, only plastic. They were made in The Netherlands. Intitial production of AR-15s was at Colt. My reference is the 1962 edition of Small Arms of the World.
I don't have any reference handy on the EM-2 other than a photo in the same book. It calls the forearm "molded" but it looks like wood.
I was wondering when plastic was first used on long guns. The Germans experimented a great deal with laminated stocks for rifles during WWII and I think ultimately began producing the 98k short rifle with a laminated beech stock, which is a polite way of saying plywood. One source said they experimented with metal butts but they otherwise only used a plastic butt on the MG34 and MG42. I don't think anyone else used plastic like that during the war.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.