2 Mystery headgear seeking identification?

[Bruce Parker (RIP)]

The pads at the ears are for headsets. I would try 19 set ones and if they fit these do look like tank helmets. Like Hanno I've never seen anything like them so have to wonder if they are trial or prototype. With all the padding yet light weigh material they were obviously meant for heads that got rattled around a lot in confined spaces (say, like tanks) and the construction would do well in hot climates. So maybe designed for the Pacific Force that never was sent?