Most certainly there were bakelite stocks made for the MG34 and MG42, but there were a lot that were wood, too.
Every MG74 I've seen had a Steyer green plastic stock on it, and many have shown up in parts kits or as surplus. I can't prove or guarantee that this is or isn't a stock for any particular gun, or for that matter, made in any particular time period. The lack of a waffenamt stamp is not definitive proof that any part or weapon is not German wartime production. And as far as that goes, the presence of the stamp doesn't mean it isn't a fake stamp and it wasn't made post-war.
Later (post war/modern) bakelite stocks have a flattened ring around the bolt in the buttplate. The buttstock plate grooves don't stop at the bolt hole, they stop at the outer edge of the ring. They metal buttstock cup also has a part number stamped inside it. Wartime stock cups have no part number, and only some of them have a waffenamt.
I give it my best guess based on my experience and am always open to the opinions of others. It seems that anyone who claims to know it all is the one who is trying to sell something that's questionable.
I offer my opinion. You make up your own mind.