Thanks, it was somewhere on Wikipedia...I did an enormous amount of research on this a couple of months ago, and unfortunately I couldn't find anything specific, even trawling through old Spanish-language archives. I'm convinced at this point that they just didn't write any of it down. And this doesn't just apply to Tejas y Coahuila, but all of the northern territories of Mexico whose borders have since been altered or rendered nonexistent by the cession to the United States. That map is an excellent find, though.
Here's another interesting one I found on a Google search, showing roughly the same border along with the old land grants...