I have a Firestone liner that has never been painted. No idea why, but a nice thing to have.
"I am a white male between the ages of 18 and 45 - everyone listens to me no matter how dumb my suggestions are!"
here's my Korean war era liner dated 1952 or 53
it was painted blue and used by the police as a riot control helmet
I removed the paint and left it unpainted
I think some left the factory unpainted
I have a liner just like yours nonpainted and the same markings