bs²w can be a second tribe/territory of which Ydʿʾb was king, or it could be a mistake for the god's name Blw. As another hypothesis we could imagine a system of acronyms: b-s²-w. It is difficult to read another line in the photo, but we can also speculate: b-s²(ʿ)[b-s¹].
Jamme, Albert W.F. 1972. Miscellanées d'ancient arabe III. Washington. 2019/07/23; https://cuislandora.wrlc.org/islandora/object/cuislandora%3A206336#page/1/mode/1up. [Privately printed]
Bron 1992: 28, photo
Bron, François 1992. Mémorial Mahmud al-Ghul. Inscriptions sudarabiques. Paris: Librairie orientaliste Paul Geuthner / Sanʿāʾ: Centre français d'études yéménites.