Yukimasa of Satsuma is known through a single signed dated the second day of the eighth month of Heiji 1 (1159), recognised as a in 1934. The published sources identify him with the Ko-Naminohira smith of the signature compendia: 「この行正は銘鑑に古波平と伝えられているものに該当するであろう」 (this Yukimasa probably corresponds to the one the compendia transmit as Ko-Naminohira). Of the inscription they write 「平治元年紀は現存する日本刀の最古の年紀であり資料的に大変貴重である」 (the Heiji 1 date is the oldest date on any surviving Japanese sword, and of great value as documentary material).
The is long, at two seven two , and keeps its original tang. The sources describe it as 「生ぶ茎で茎先鍵形ごころとなり腰反り高く踏張り強くついた姿である」 (an tang whose tip tends toward a hooked shape, with a high -zori and strong ). The is a tightly packed mixed with , with , and fine lying thickly over it. The temper is recorded as 「刃文小沸出来の小乱二重刃ごころあり、砂流しかかる」 (a in with a suggestion of , and with ).
On the the tang also bears the name Kuniyasu, whose meaning the sources leave open, suggesting it may be the name of an owner. The blade is published in the Satsuma To Meisakushu and the Toei Tekiroku. On this one work Yukimasa stands as a late smith of the Naminohira line in Satsuma, his dated combining a deep -zori with the -mixed and -based that the sources record. He is distinct from the smith of the same name.