Thanks a lot Markus for your answer. You were right at 100%. There were other pctures that my friend forgot to send me and his question was far from the one I initialy planted..
you are going to see all pictures. He has been asked to make a shirasaya for this blade. He has two options, the first one is to amend the existing one (with the sayagaki) , the second is to make a new one. The existing one has been made by someone who seemed not to have been in the craft. The nakago/habaki is definitely not in the tsuka alignment. (See second picture)
As the sayagaki has no value in itself, I told him to make a new one and to send back the old one to the owner so he can store it away. I very doubt that this shirasaya was original to the blade.