Yves,
this is called TEKKOTSU (= "iron bones"). It is a special feature of some TSUBA makers or schools. There is some discussion about what it is and how it is done. Unfortunately, most writers have no metallurgic background and try to approach the subject from the aesthetic side which does not lead far.
Your TSUBA would be classified as HEIANJO style, in this case an earlier one with HIRA ZOGAN technique (flat inlay), most of which is still present. There are information that some TSUBASHI working in that style did not make the TSUBA plate themselves but bought them from more specialized smiths to decorate them afterwards. This may be the case here.
Attached is an image of a HEIANJO TSUBA with another decoration style.