I recently purchased a new (to me) shirt of mail. People who know better than I have helped me to date it to the 15th c.
It includes a little crotch flap.
I have marked the main body of the piece to show how it was tailored to actually fit a human body (something that is often skipped in modern shirts).
Here is a picture of the new shirt along with another one. It shows the tailoring in the back of the two shirts:

The new shirt is on the right. We can see that it tapers out at the shoulders to allow room at the shoulder blades, then back in at the waist, and finally back out so that the flap can fit to the buttocks.
This is the page describing the new shirt. It includes images of the rings close up and shows the additional tailoring in the chest:
https://european-armour.com/M-18.html
Comments welcome.
Wade