Odd number under back of M35 Police helmet, any ideas?

[Larboard]

This isn't my helmet and it isn't for sale. I'm posting here as a favor to the owner to try and find out what the numbers 305 mean, although I told him it was unlikely that anyone would know for sure. It looks like a double decal M35 police with the eagle and wreath decal scratched off for what reason I don't know.

He swears it's a bona fide family bring back, as brought back.

[LeonardoL]

Just a number so the owner can ID it. These numbers were usualy usualy found on gasmasks but were used on helms and other equipment too.