The Johnson roughback sheath doesn't mean much contrary to what you'll hear at your local gun show. I wish I had $5 for every time I've heard "yes sir, that's an old one...look at that Johnson roughback sheath".The Johnsons made sheaths for Randall from about 1961 to 1991 and the vast majority are roughback. Only a very few of the first ones(split back) and then the last ones(smooth back) are not roughback.
The shape of the Model 9 doesn't vary after about 1982 but before that there are several variations. IIRC the museum at the Randall shop has 10 or 12 different shapes of Model 9's from the earlier years.
The length of your knife says late 70's to early 80's and that's where I'd put it if asked for a WAG. With your knife, I really can't think of any way to get closer to its age.
Bob
