That is true, but at the same time it didn't necessarily have to be bright yellow. Also, you see a lot of helmets and uniforms with colorful insignias on the outside. A unit mark is normally used to identify who a soldier belongs with, and it's useless for that purpose if you can't do it without taking his helmet off and remove the liner. However, I have heard that they did do away with just about all identifiers like that in the Pacific theater, to the point that medics weren't marked with the red cross and everybody was told not to salute the officers. This could be a possible reason to hide the unit mark inside like that.
I'm almost totally uneducated on the Marine Corps organization, so is 6th Marine Regiment, 2nd Division the final verdict? If so, I assume that they were deployed in the Pacific theater?