See N.B.T.H.K., Juyo token to zufu, vol. 3, no. 41 for an example of this shushu-uchi type of signature.
With the mitsu-mune, configuration, kitae, ji and hamon of this blade, it is evident it was made as a copy of Sadamune. Dr. Homma illustrates two examples (Showa dai meito zufu, nos. 111 and 115), a National Treasure and an Important Cultural Property, both with carving which display the same characteristics that Yasutsugu sought to recreate.