It was estimated that maybe 300 KBN-36X were made in the 1975 time period. You have a neat gun there and I do not doubt that it was shipped with rosewood grip panels, but how many owners even think to check the grips for type of wood? I'll bet that quite a few (but still a small percentage) were shipped with rosewood as original. I have seen several stainless steel single-Sixes as NIB from this period (especially star marked guns or the KNR4 and KNR9) with rosewood panels. I also know of a consecutive pair of 3-digit Old Army's (#633 and 634) that have rosewood on them as original.
Send me a PM on your serial number and I'll be sure to make the new note in the "Reference of Ruger Firearms". If you don't mind, I'd like to see a pic of the gun also with a closeup of the grips, just to make sure they are from the correct 1974 era.
As to extra value, your gun would be interesting to a collector, though there probably aren't too many collectors any more who recognize the collectibility of the 1975 era KBN36X. There were also 15,000+ identical pair of rosewood panels used on the Colorado Centennial Single-Six, so not even Ruger would be able to tell anybody what panels these Blackhawks and stainless Single-Sixes were originally shipped with.
Chet15