I have a small collection of military swords, I am no expert on Japanese swords, however looking at the pictures it would appear to be possibly world war II maybe earlier, it is hard to tell with out seeing the sword itself and any markings. I would not recomend doing anything including cleaning until it was appraised by someone knowledgable in this type of sword. If you were to recover it you would stitch the leather like a sleeve or a glove the you would force it over the Scabard until the scabard was completely seated in the sleeve. It would then be laced above the lanyard ring ( not the correct terminology but you should get the meaning).