From what I know the first models had blued recievers and stainless barrels. Not many of them made in the beginning. The target grey came next and remained for years.
The matte stainless is on the newer target models now and I never paid much attention to wether or not they are also done in matte blue. But that would be my guess. Or at least some were done in matte blue when the 77 hawkeye rifles went to the new finishes. Maybe a check of the serial number with Rugers site to get an idea when it was made can give some insight to that.