Southern Ryukyuan is one of the two main subgroups of the Ryukyuan language family, consisting of Miyako, Yaeyama, and Yonaguni. One of the notable features of the subgroup is word-initial fortition of Proto-Ryukyuan *w into b, exhibited by all three members; in contrast, only Yonaguni underwent *j > d, while the other two languages have retained the glide value for *j. Due to the asymmetry, it is not entirely clear what value should be reconstructed for the Proto-Ryukyuan glides at the Proto-Southern Ryukyuan level. To answer this question, this paper investigates late fifteenth- and early sixteenth-century Korean transcriptions of Miyako in the Veritable Records of the Chosŏn Dynasty. In particular, it is argued that the initial [(ⁿ)ʑ] found in transcriptions of the name of Irabu Island faithfully reflects Proto-Miyako *[ʑ]. This evidence supports the reconstruction of Proto-Southern Ryukyuan *β and *ʑ as the reflexes of Proto-Ryukyuan *w and *j.