He may be talking about parts such as extractors. I have usgi extractors that are made by Parker that are not marked. The only marked thing about them is the package they come in which has the Parker name, but when you open the package, the extractor is not marked at all. Parker may be post-war, and that may be why they are not marked. All parts made during the war may have been marked, I'm not really sure, that's something that the experts would know I'm sure. Was all parts made during wartime marked?
There are some smaller parts but for the most part all genuine USGI WWII carbine parts are marked.