Tom, sure, if all-original, it would be made right at about '84 into '86, when the 'all thin' spacer stack was introduced, and the tightly stitched Johnson Roughback Sheath style was being phased out in favor of the looser stitched version. But, with regards to the sheath; older sheaths could languish in sheath bins at the RMK Shop (first in, last out rule), buried under newer arrivals if you will. And, while not (as) commonly used by son Johnnie Johnson, as was by his father, Maurice, the machine that produced the tighter stitch was still used on occasion, such as when the looser stitch producing machine was down for repair. And/or, the sheath might've simply gotten swapped out or mixed up with that of another, older knife, in the past. That's pretty common, too.
The key is almost always the spacers, that indicates when the knife was made, and as such it can't be older than when a feature, in this case, the spacer type, was introduced (spacers, hilt/handle material, blade stamp type, compass, etc.) - that said, as RMK's most popular model, in the most popular blade length, I'd wager not many of that sheath type languished for any length of time; thus the mid 80's date given. Still, exceptions...