Regarding 0 vs 00 designation, the Japanese made a conscious decision to start at 0 and use single digits until they got to 10.
That designation system was supplemented by the Ki designation used for various types of aircraft.
The entire Japanese procurement and designation system was a boondoggle in large part because the Army and Navy were so competitive and often very much in conflict with each other.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.