For me, there are 2.
There is this little prong used to hold heads, like the ones in the new armor pack ones for the green and black one, but it didn't curve. So if slotted on a spartans back, they could hold their helmets on their back while showing their face.
The second one is the Reach Jetpacks. I REALLY wish there was a reach era ODST/Spartan pack that came with 2+ jetpacks.