I have a no-pictures-available reply …
Staying at my barn is a mare who was bred about two years ago. The colt lived about two months, so she was still lactating pretty freely at that time and while generally jumpy, she would let me nurse on her.
She's back, and she still lets me nurse on her … and one teat is still lactating. A very little amount.
Granted I had tried every other day, on average, to suckle because mare-with-titties amiright? But even the first time, I was able to extract the taste of milk, if no perceptible fluid. Last time I tried I got, I dunno a quarter of an ounce of milk from one side? Nothing from the other.