The middle part was the name of the original owner. It's been crossed out, and a new name written to the left of it. The right side looks like a rank
囗兵中尉
代藤忠良 Lt.
SHIROFUJI Tadayoshi.
Not sure about the ones in red. The first one is a bit smudged, and the second one doesn't feel right.
The rank isn't so important because the "Lieutenant" part is clear enough. It's the family name that bugs me.
SHIROFUJI (using those characters) doesn't seem to be a plausible family name in Japan. It doesn't pop up in any searches. I'm not 100% confident in the first character. It looks like 代 but with some extra bits in the middle. It could possibly be 城 (which is coincidentally also pronounced "shiro"), but somehow the left side doesn't look right.
Looks like the name in the middle was the original owner, but that name has been crossed out with two vertical lines.
That name might have been UCHISAWA Katsuhide (内澤勝英) or OKASAWA Katsuhide (岡沢勝英). He may have been the lieutenant.
Anyway, those are my best guesses. And yes, even if you could find the person's descendants, the gun couldn't be imported into Japan. It would be impounded by customs, and would cause a serious headache for the family. And, I doubt the family would want the item anyway. If it were a photo book or a soldier's pay book it might have sentimental value, but a gun is not something that they would want.
On the other hand, if it is a working Nambu pistol, you might be able to sell it at a gun show here in the States.
Legacy Collectibles has Japanese Nambu pistols for sale from World War 2 We buy and sell all models of Nambu pistol from the Papa Nambu to the Type 94!
legacy-collectibles.com
You might try posting it on the Japanese Militaria section at Warrelilcs.com to see what they think.
Edit: I posted this just after you posted the two messages above. My gut feeling is that even the case would not be particularly welcome or interesting for them. You do see memorabilia such as flags and swords returned to Japanese families from time to time - so not all families feel the same. But a gun holster by itself I don't think it would have much sentimental attraction.