That is what I have been hearing about the markings.

Once again a shameless plug for my website, my Dad was on Roi Namur, Saipan, Tinian and Iwo Jima, I did a website for him and his men HQ224USMC Honoring the men of the HQ Company where I have battle pictures and pics of the islands today.
I am also putting his Battalion CO's auto biography Col Richard Rothwell a China Marine and one of two remaining Battalion CO's left.

We had some of Dad's ashes scattered on Mt Suribachi, pictures on the website and he was featured on Battle Rats: Iwo Jima with the ash ceremony

Jeff Padell USN

PS my nephew is a LTjg NFO back seater Eli Padell USN - The Padell Genealogy Website