Uku writes the vowel u and takes the numeral four hundred. Its name is one of those whose sense is no longer recoverable with confidence, and it stands at the head of the run of opaque names that occupies the rest of the alphabet.
The vowel is a back rounded vowel and it is common in Slavonic, but the way the two alphabets came to write it is not straightforward. Greek by the ninth century wrote that vowel with a pair of letters rather than a single one, and the Slavonic scripts had to decide whether to follow. Cyrillic did, producing a compound sign that was written as two letters and later fused. Glagolitic's treatment of the same sound is described differently by different authorities, and the forms in the manuscripts do not all agree.
The numeral four hundred follows from counting the Glagolitic letters in order, this being the twenty second sign, and it agrees with the Cyrillic value inherited from Greek.
The letter is common enough that its forms are well attested in both the rounded and the angular hands, so the uncertainty is about its history rather than about its appearance.
It is worth being explicit about the shape of that uncertainty, because it recurs throughout the second half of the alphabet. What is well recorded is the repertoire of signs the surviving manuscripts use, since the manuscripts are there to be read. What is not recorded is how the alphabet looked when it was made, because nothing survives from the mission itself and the earliest books are a century or more later and were produced in a different country. Statements about the original inventory are reconstructions, and the atlas marks them as such rather than presenting the reconstruction as the thing reconstructed.