Period: In the opinion of Bob Benson, the blade is not that of the well known shinto smith, but most probably Koto (900 AD-1530 AD)
Mei: Sukenao. Gimei. Of course, if it proves not to be gimei, I'll refund your money.
Sugata: Hira-zukuri, iori-mune, uchi-zore which is typical of a Kamakura tanto (1185–1333).
Overall length: 10.88 inches (268.73 mm).
Nagasa: 7.63 inches (188.46 mm) long.
Nakago: Ubu, 3.25 inches (80.28 mm) long. One mekugi ana; the yasurime are katte sagari with kesho yasuri. The nakagojiri is ha agari kurijuri
Kissaki: No yokote. The boshi is kaeri-fukai.
Moto-haba: 0.88 inches (21.73 mm). Saki-haba: 0.63 inches (15.56 mm). Moto-gasane: 0.22 inches (5.4 mm). Saki-gasane: 0.13 inches (3.21 mm).
Sori: Uchi-zori 0.5 inches (12.35 mm).
Blade condition: In perfect condition and perfect old polish; no flaws at all.
In shirasaya with copper habaki.
The sugata indicates an early Kamakura blade. However, the yasurime are indicative of a later period, and the tang is longer than might be expected. This suggests that the blade has been shortened slightly, without making the tang o-suriage. All the indications therefore are that this is a 12th-14th century tanto that has been subsequently altered in the Shinto period and given a false mei. Even so, it is rare to find an early Koto blade in quite this good a condition. A very nice piece; I may make koshirae for it.
My guess? It's a Yamato Tegai or Yamato Sue-Tegai tanto of the 13th-14th century. It is a pity that it has a gimei. However if that were professionally removed - which is possible - it might well paper.