hey quite a nice find, most Katanas have but one hole in the handle area, this is where a bamboo peg was put in to retain the handle onto the blade.
The center hole looks newer than the other 2, i say that due to the discoloration..?
The markings on the handle, if 500 yrs old would have been the owner or family name and in Japanese, not chinese.
Where is the remainder of the handle and guard, do you have it??
Looking at the blade is there a wavy line running down the whole length of the blade on both sides?
a closer look at the blade would be nice also..
tnku