This is the retaining strap and bracket for an advanced combat helmet and the mounting bracket for an M4, for PVS 14 monocular and the ATN 6015, would likely work with other monoculars that are able to mount to helmets but you would want to check your brand.
This mounting equipment seems to go for really insane amounts on eBay for some reason, not sure why.
Looking for $100 for the everything you see, if you only want/need the helmet assembly or the weapon assembly let me know