A Ruger collector once told me a little about the brass grip frame old models. From what I can recall...
Models with brass grip frames were supplied in very limited numbers, and I believe the brass grip frames were used for a short time because of a shortage of aluminum ones.
Ruger also sold small numbers of individual brass grip frames.
The brass grip frame ones weren't really marketed as a distinct model, so production numbers weren't well kept. Subsequent to those models being built, at least one individual was able to examine Ruger records and "authenticate" certain models, by serial number, that were built and shipped by Ruger with brass grip frames. I believe there might have even been some sort of model number suffix or marking on the original boxes indicating a brass grip frame on some of them.
Original models built and shipped with brass grip frames will bring significant premiums to Ruger collectors. But realizing the premium also necessitates some evidence that Ruger actually originally built a given Blackhawk with the brass grip frame. And I'm not knowledgeable enough to know how that's accurately determined.
Since Ruger also supplied individual brass grip frames, some owners and gunsmiths replaced the aluminum grip frames with brass ones on some Blackhawks. Those will not bring nearly the same premium as one originally supplied with a brass grip frame by Ruger.
There may also exist some brass Blackhawk grip frames that were supplied by other companies.
I was hoping someone with better knowledge than myself would chime in with more accurate information. But that's the best I can do, and I can't guarantee the accuracy of the information I provided since it's second hand. Unfortunately, the individual who originally spoke to me about the subject is no longer alive to ask again.
Edit: After posting I did a quick search, and there does seem to be some additional information available. Give it a try.