Three of the four Tibetan vowel signs carry names that can be read against ordinary words. Zhabs kyu contains the honorific word for foot and describes a sign that hangs below its letter. Greng bu is read as the upright one, from a verb meaning to stand. Gi gu is glossed in the tradition as well.
Na ro is the exception. Readings are offered in the grammars and in the commentarial literature, and they do not agree with one another. This pass found no basis on which to prefer one, and no source it had checked that established an answer.
So the atlas states the sign, its value, its position and its behaviour, and declines to gloss the name. That is a deliberate choice rather than an omission. A plausible etymology nobody has checked is worse than a gap, because it will be repeated, and a reference work that guesses in one place cannot be relied on in the others.
The gap is recorded as an open question, held internally until it can be attributed to someone who posed it, with a note of what would close it: a study tracing the term through the earliest Tibetan grammatical treatises and their commentaries, establishing when it first appears and whether an early commentator glosses it, rather than a further proposal built on the modern shape of the words. That is a philological task, and it is a small one. It has simply not been done here.