my thinking is: the sword passed Juyo so it is Juyo. Maybe it would not pass now as standards change but the same would be true if the papers were not lost. With Hozon and TH if the papers are lost there is no proof they past shinsa but with Juyo the blade is recorded in the annual Zufu or book published that shows all Juyo from that year. These books are owned by a number of people. If i had a question i would find someone with that volume, look up the blade, then CAREFULLY compare the blade with the one in the book, check the nakago, boshi, hamon (the NBTHK shows these in the book) to verify that the sword is the one published, if so then i would not have a problem, the paper is nice to have or display but the sword is what you are buying and collecting (unless you collect papers)