Unknown M1 helmet shell stamp (previously used by the Japanese police?)

[tetsubo]

Hello,
The image of the stamp for the M1 helmet shell attached below shows "H34H", and as far as I've researched, I don't know which manufacturer it was.
Would you tell me about the maker of this stamp?
This shell has a swivel bail at the rear seam.
From the second image I believe this helmet was previously used by the Japanese police, but I will post an explanation on that later.
Best regards,

[tetsubo]

In the 1960s, when student movements became radicalized, Japanese police needed heavy equipment to compete them.
It is well known that Japanese police used Type 90 helmet, but M1 helmet were also used.
It seems that each prefectural police department purchased a surplus M1 helmet , and welded a triangular metal fitting to the rear of the shell to create a chin strap configuration similar to the Type 90 helmet.
As proof of this, there is a modified version of the Austrian M1 helmet, as shown in the attached image below.
Regarding the Japanese police M1 helmet, there is only written in Japanese, but this page has more information.

[tetsubo]

I think my helmet was procured in a hurry because the triangular metal fittings are poorly made.
See here for reuse of the Type 90 helmets by the Japanese police.