The last bit of Thanissaro selves and not selves is of note where he talks of what is beyond self and not self. So for grades of practice, an advanced one is to apply not-self across the board. Maybe saying "a self is un-findable" seems a bit too much seeing things in terms of self, as no-self. Its more about dropping categories of identification and obsession. And Ajahn Chah "nor anatta" is about talk as nibbana as beyond even not-self (?), as an ultimately dropping of categorization, as beyond discrimination.
Many of the forest ajaans have emphasized this point in their teachings: that in the attainment of awakening, you put aside both self and not-self. Several years back, there was a controversy in Thailand as to whether nibbana was self or not-self. The issue was even argued in the newspapers. So one day someone went to ask Ajaan MahaBoowa, “Is nibbana self or not-self?” And his answer was, “Nibbana is nibbana.” That was it. He then went on to explain how self and not-self are tools on the path, how both are put down when the path has done its work, and how neither applies to the experience of nibbana. Ajaan Suwat, one of my teachers, also said that when you’ve experienced deathless happiness, you don’t really care if there’s something experiencing it or not. The experience is sufficient in and of itself. What we’ve been describing here is a special kind of consciousness that lies beyond the aggregates: The texts call it “consciousness without surface.” Once it’s been attained, then freedom is never lost. The mind no longer tries to define itself, and because it’s not defined, it can’t be described.