OK! The rifle was made at the Kokura Arsenal sometime from 1939-1945.
Your pictures aren't clear enough to make out the series mark (Just in front of the serial number) or to see if there is another mark after the Kokura mark (the 3 lobed mark after the serial.)
Kokura was one of the main arsenals before and during WW2.
I've attached a list of the seriesmarks that should appear om the Type 99. The serial numbers ran from 1 - 99,999 and then a new series was started. This will give you an indication on just where in the 1939 to 1945 your rifle was produced.....