Was it on #43 or #4 ?
It's not either one. Can't see the number clearly, but it looks like it might be 86. He starts on the front line, retreats, and make his illegal block right on the 20 yard line, just inside the numbers on the top of the screen. If you watch the slow motion part of the replay, the fouls occurs at the 0:35 mark.
Question to Texas officials: under NCAA rules, the number of the fouler is announced. That was not done here. Is that a difference between NCAA rules and Texas rules?