Problem is, the Type 38 was made, in almost unchanged form, for nearly 40 years. Probably more than 15 million were made, even though that site says 6.5 million. I think that figure is dramatically low.
It was not uncommon for the serial numbers to be repeated, so pinning down a date of manufacture can be almost impossible.
You can get a clue as to relative date of manufacture from the quality of the materials. Things started to go downhill fairly quickly after 1939 as Japan became more and more pressed.