From my understanding, Iwo Jima and Okinawa were the only battles that had 3 Marine Divisions take part in at once. For Iwo Jima, that is a lot of logistics for A LOT of Marines, and logistics needed to know who was landing where. The Marine Corps leadership had learned from previous battles such as Peleliu and Tarawa how chaotic the beaches could be on the first day; with a battalion/regiment's ammo/supplies being deposited where it wasn't supposed to be, in addition to Marines from different regiments overlapping and veering off from objectives. (The Army was able to wear shoulder patches on their jackets to distinguish them, the Marine Corps didn't). The Marine Corps still kind of operates this way. During big exercises where there is overlapping Battalions or Regiments, one unit will wear woodland cammies and the other desert cammies to tell who is who. The Army will just wear a patch.

Outstanding shedding some light on this topic.

In addition, UNIS markings were used on crates for shipping. If they used White lettering then the regiment was Infantry, Red was for Artillery and Yellow for all others. However, black was often used in place of color because it was the paint most available.

...as far as the the top # outside of the symbol, 2 would be Private First Class...

I had no idea these things existed....

I know it's a slim chance, but I was hoping that someone knew what the marking codes for the 1st Marine Division would be. There seems to be a lot of variation between Divisions, and I've found varying sources. Unfortunately a lot of the Forums about UNIS marking have been archived or are gone.

The 1st MarDiv and 3rd MarDiv unis markings are actually flip flopped. Horizontal diamond is 3rd and vertical diamond is 1st. I have several unis marked items with the horizontal diamond and they are all ID'd to the 3rd MarDiv.

Do you know how they applied them?