I hear S join together with Rand EA and W and M to declare an oath. (50-51)
These lines personify the runes by saying they declare their oath aloud. Since an oath was a verbal contract between two people, the fiction of their "speech" is necessary to give this one legitimacy. They speak "for" the lord, making us wonder: Is it really possible for writing to speak for a person, to declare a vow in his place? Runes are mysterious; their meaning is often ambiguous. Their presence here reminds us of how all writing is a bit mysterious: yes, it can bridge the distance between two people, but it can also be impossible to interpret.