Technically this helmet does not belong in this section of the forum as it is ultra high molecular weight polyethylene, not steel or kevlar
It should be noted that the IHPS in the above picture is an old prototype from 2017. Notice it doesn't have the holes in the top of the mandible. The more recent picture of an IHPS below has holes added.
I don't know for sure as it hasn't been mentioned in the press releases so far, but I am fairly certain these are to prevent the mandible and visors from trapping secondary shock waves from explosions which significantly increases pressure inside the helmet. In the case of Naval Research Laboratory tests on the CIPHER helmet prototype they found a mandible and visor more than doubled pressure inside the helmet. (source)
This picture does a good job of showing how the mandible clips onto the side of the base helmet.
I am confused by the screw on the mandible mount because the PEO SPE manager Lt. Col. Whitehead has said that the only bolt hole in the IHPS is for the NVG mount, perhaps the crew does not go all the way through?
The applique plate may be attached with a strap that goes over the top, but this strap can been seen on IHPSs without the applique plate, so it may be to hold the visor/NVG mount in place.
It looks like the IHPS will have nape pad armor like that used on the ACH, it may be a new design or NSN as I looked up the NSN for ACH nape pad armor and no contracts have been made since 2013, although it is possible that contract is indefinite and they are still being manufactured under it..
The chinstrap is different from the Ops-Core Head-Loc used on the some ACHs and on the ECH. The rail can be removed from it's mount. I am still unclear entirely how the camouflage cover attaches. This one has "velcro" on the helmet shell under the cover, but other examples do not.
The US Army Acquisition Support Center says the IHPS is 5% lighter, but does not specify to what, I assume the ECH. It also says the helmet has passive hearing protection but does not elaborate. They also also say the ballistic applique allows the helmet to provide protection from rifle rounds, but ACH's have already stopped 7.62x39mm in combat, and the the IHPS was unveiled at a ceremony where SSG McQueen was presented with his ECH, which is made of the same UHMWPE, which stopped a 7.62x54mmR round at 20m square on.
Looks like the cover has the same IR squares with covering flaps, but has more "velcro", the corners are rounded which makes me think they are not for patches, whcih means they may be to attach the ballistic applique, but this seems somewhat un-sturdy to me. It also looks like there is a cord and lock on the back which is probably to adjust the strap that goes over the top.