Dating a japanese sword
I am not sure about the oiling of the nakago, but certainly were you to find yourself in a battle, or having been in a river or other wet circumstance, it would have been easy for a samurai to remove the tsuka and dry off the tang.
I would say that there are no homogeneous criteria to be applied because of the sheer number of variables (steel used, cultural approach to maintenance, period of making, site and type of storage, quality of maintenance in the near past, type of wood of the handle, quality of the craftmenship, ). Sorry the photos were not good enough to make a determination on the tang age.
At the end of the process the tatara will have consumed about 10 short tons (9.1 t) of satetsu and 12 short tons (11 t) of charcoal leaving about 2.5 short tons (2.3 t) of kera, from which less than a ton of tamahagane can be produced.
The swordsmiths will carefully break the kera apart, and separate the various carbon steels.
As with many complex endeavors, rather than a single craftsman, several artists were involved.
There was a smith to forge the rough shape, often a second smith (apprentice) to fold the metal, a specialist polisher, and even a specialist for the edge itself.
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I've seen purported photos of Japanese Katana from 16th & 17th century with file marks still visible on the tang and on a Chinese sword the file marks are likely to have been eaten away after 100 years.