Gerald is right about the liner (I have mine in the loft, the Dutch Army used them well into the 1990's ) so the internal suspension and head band etc can not be used with these helmets. You can however still put the strap that is usually worn over the front lip in place, as that strap is attached to the steel outer helmet. To put it down as an accessory on a vehicle or such it still works. Just don't use the liner straps.
And no, you can't wear the outer helmet on it's own... unless you have
very luxurious hair...
Henk