Knurling could not...and cannot...be added. I was at the shop today and Scott and I jawed a bit about #18's. We generally agreed about dates for knurled handle. Neither of us remember 1970 vintage with knurling but both agree early 80's.
As for this example? The blade stamp looks newer than early 1980's. A check of the inside of the handle will show some valuable insight as to the year. If the threads for the tube start immediately at the throat...the knife is earlier. If there is a recess (space) on the tube, prior to the start of the threads it is later.
Lined caps were left in stock and will fit some of the newer tubes as the space allows for some "jiggle room" before the threads engaged. As for the tight-stitched JRB sheath? Could be somebody trying to make a newer knife look old (er)...
Best, Capt. Chris