Konnichiwa Stuart Liptrot-san!
I think it's a Japanese army helmet "九〇式鉄帽:Type 90 iron helmet".
However, the flag print and lettering were all added later. Even the blue color on the surface.
The original color of the surface is olive, the same as the color of the interior.
Yes, post ww2 police helmets are blue, but do not have the star marking on the front. And police helmets are blue on the inside too. For this reason, this helmet is not for police use.
If this helmet were to be used for civilian purposes, the star marking would definitely be removed. For this reason, this helmet is not for civilian use.
The original owner/Harrison? may have acquired this helmet on the battlefield. The second owner then acquired the helmet with the intention of reselling it.
The second owner likely knew the helmet was for military use and added flags and lettering to it to sell it for a higher price.
Unfortunately, the second owner did not know the original color. And he/she may have mistaken the police blue color, which was common at the post ww2, for the original color.
Then, in an effort to clean it up, the second owner repainted the helmet – the wrong color blue.
The result was a blue military helmet like this one, I think.
NANGI