Bob is correct, Mary. His unit info would read (from smaller component to larger) H Company, 2nd Battalion, 23rd Marine Regiment, 4th Marine Division. Typically, a regiment in the Corps is referred to as "Marines", as in "23rd Marines". So, you would say he was in the 23rd Marines. This,incidentally, would indicate to everyone familiar with the Corps that he was in the 4th Marine Division. That is because there was only one 23rd Marines in the Corps, and it was one of the infantry regiments in the 4th Division. It is not customary to refer to a division in that manner, so referring to his outfit as the "4th Marines" would be erroneous. (The 4th Marines were one of the infantry regiments in the 6th Division.) H Company and 2nd Battalion were not unique. Each regiment had one of each. H Company, incidentally, was a heavy weapons company, which included machine guns and mortars.