Eric,
Couple of things: 1st, call me as I have RMK "stuff" for you!
2nd: 1990's, as well as 1980's showed these all-stainless Model #18's being offered, although on a very limited basis. I had, at one time, "6" examples in my collection...with no 2 the same! I also had one stolen and GTR agreed (after I showed him the police report to make me one in the late 1990's. Floyd Byrd supplied the butt caps for all of these knives, including the stainless #18, Minis and even made some stainless steel "skull-crusher" butt caps for the #18. His later butt caps (1990's vintage) featured "knurled" stainless steel caps, verses the lined cap shown on yours. All were in JRB Model "C" sheaths, to the best of my knowledge, although the later ones were sewn on Johnny's machine as was evident by the wider stitching.
Hope this helps and yours is a perfect example! Well done!
Best, Capt. Chris