Probably, I incomprehensibly explained

It was necessary to put the link:
https://www.ebay.com/itm/ANCIEN-COUT...wAAOSwx0FdZCQw
This choora dagger, judging by the description, is declared as ancient. Beautiful floral carving on a blade is a modern work. I will explain again what I wrote a little earlier. If the old master made this carving, he would decorate the T-shaped spine this choora dagger in the same style. The photo shows that the T-shaped spine this choora dagger is decorated very roughly. Old masters did not allow such stylistic differences.
So we are dealing with a fake ... As I said before, there are fakes of a very good standard

I didn’t say anything about the hilt

Perhaps my bad English is to blame. Sorry for not being able to explain right away...
For me personally, modern work (in my understanding "modern work" for an object that is 100 years old is 10-30-50 years old) on an old blade (even if it will be very good) - reduces the value of the item ... For me, such an object will be fake.
Marius, I agree with you that Japanese swords are usually made by a master swordsmith and occasionaly decorated/engraved (horimono), sometimes at a much later date by another master. That doesn't make them fakes, nor does it decrease their value.
But if I make an engraving on an old Japanese blade - what do you say about this?)) Will it be a fake?