I just checked my Texas hunting license, and I don't have that stamp either. It's apparently a new requirement, but when I requested my license at the beginning of the season, I said that I wanted to hunt deer, dove, and quail.
I assumed that's what I purchased, but obviously I didn't. I haven't hunted quail this year, although I had several opportunities to do so, and would have assumed I was fully licensed to do so.
If you bought the Supercombo, you got a quail stamp.
Same deal with Cheney: His staff asked for all permits and they forgot to give him the quail one.