It is a police helmet, its a M40, and the police decal is the "boarded decal" its a nice example, well worth looking for a liner etc, the number shown is a prodution number IIRC, the size will be two figures with a prefix letter or two found at the side ( on the inside of the rim) you need this to buy the correct size liner, some originals show up plus the rivets on ebay, as for price couldnt say, I dabbled with helmets about 7 years ago, but it would fetch a good price because of the police decals and the condition even with no liner I a collector would snap it up.