Need help Identifying helmet and liner

[Kim]

Hi! I’m new to this, I decided to check into this forum since I recently found family military goodies. I have a helmet, that is gritty texture, no grommets on the outside, (which most of them I see have grommets coming through to outside of helmet), the liner I have more info on. It is a high pressure liner, labeled Inland #34, It has a stenciled name on the front in parenthesis “Oakie” with the number A131. Not sure what that is even for. Then there are numbers on the back and a name stenciled as well. The back has the name RIBKEN 551727.
My question is, What era do you think they were from? I believe they were my Father in laws, he flew Helicopters in Vietnam.

[Kim]

Here are the helmet pics

[ruddersrangers44]

Hi Helmets never had grommets on them only Liners,also there actually called eyelets and were for attaching an insignia or rank,the Liner is a late 1942 Inland with unpainted steel A washers,the shell is post oct-1943 swivel looped variation,can't help with the Liner markings.