It is just the superlative prefix. There is something in paragraph 3.3 in https://books.google.cz/books?id=SuR8DwAAQBAJ&pg=PA1546&lpg=PA1546&dq=proto+slavic+superlative&source=bl&ots=HLHzr1gAgP&sig=ACfU3U0Xhni041yaBb-ZuDPX7JHdNz-s1w&hl=cs&sa=X&ved=2ahUKEwjzw8_1yLnjAhXEaVAKHYPbBQEQ6AEwA3oECAYQAQ#v=onepage&q=proto%20slavic%20superlative&f=false Your guess may not be that far (na is the same as current Czech na (on). (j)i meant what is on in Czech (he) but perhaps it is a meaningless particle here? I do not know.).