Marine HBTS dont have all the pockets that the Army 1s did, they have 1 left trouser pocket on the trousers, and 3 pockets on the top, 2 hip and 1 left chest (where the marine crest is)
A good film for looking at the HBTs is prolly Windtalkers (which is very cheesy but good selection of kit setups) or Flags of our fathers (all HBTs)
From the Thin Red Line
Here's me.... you'll have to excuse the excessive headwear! 
nah they're deffo Marines, the island in the film was Guadalcanal.
Most of the battle was fought over an airstrip, which if captured could have been used to stage deeper bombing runs into Japanese Terratory (similar to Iwo Jima and numerous other Island raids)
You know me... fountain of knowledge 
I do like the bit in the film where 1 of the guys takes out a number of Japs on the ridge, and then shouts to his mate "I GOT 1, I GOT 1" and subsequently half the Japanese artillery comes down around him! hehe silly american!
I always thought the thin red line is about an army unit not marines. The marines staged the inital landings and took Henderson airfield but the army then took over to push the Japanese off the island. If you say they have marine uniforms though I'm not going to argue as I don't have a clue in that regard.
Now that is an interesting point, I would have to watch it again! The uniforms look like a mix of Marine and Army, but they were part of the initial beech landing at the start of the film if I'm not mistaken? Where they go ashore with hardly any resistance?
This is a sysnopsis of the film, but its not an official 1, the official 1 just says C-Company, no mention of what forces.
"It is 1942 and the battle for Guadalcanal has commenced. The Pacific island must be seized and secured by American troops so that the island can be used as an airbase from which to attack Japan. Lieutenant Colonel Tall (Nick Nolte) leads a battalion which is about to land on the island as part of the invasion force. A career soldier, he is anxious to enhance his reputation in this battle. As the marines file into the landing crafts, he tells the commander of C company, Captain Staros (Elias Koteas) that he is confident in him to get results."
You're probably right. Its been a while since I've seen the film; I can't say I even remember the beach landing bit.
In which case they prolly are the army then. It does say in the synopsis that its based on fictional engagement... who whoever really.. I don't think you'd go far wrong having a Marine impression for the Pacific theatre, (especially for things like Guadalcanal, Iwo Jima, Saipan etc.)