I think it's a California Police helmet. It's a size 7, so I think it's probably about S size. There is a B-T 4 stamped on the earpiece. It will be vintage, so we appreciate your understanding.
I think that the condition is probably good for a helmet in the 60's. This item is in the category "eBay Motors\Parts & Accessories\Apparel, Protective Gear & Merchandise\Helmets & Accessories\Motorcycle & Powersport Helmets". The seller is "shopping-jp" and is located in this country: JP. This item can be shipped to United States, all countries in Europe, all countries in continental Asia, Mexico, Canada, Australia.