If you notice the blue paint isn't as weathered as the white. I believe this is like that by artificial ageing.
Just my opinion.
It looks good to me. and under the circumstances it was not sold to fool by the sounds of it.
If I were you I would contact the seller politely and ask them a few questions about it, you may find out who the original owner was with a bit of luck. I've did this before explaining that I like to keep any history with the item in question. some people are kind enough to reply, some do not.
best of luck
If Troy from USM forum said it is a good helmet I would lean towards with him. Marty
Fortune favors the brave 644th td