My 1956 and 1957 flat top Blackhawks has black "plastic" grips. Are these not factory?
Your grips are factory original. They look like nice walnut to me, not rosewood.
The very first gun out of the factory was probably the gun that appeared in the American Rifleman Dope Bag Report, serial number 14 I believe it was. It had walnut grips on it. Before Ruger started shipping guns it was determined that the checkered black rubber grips were not nice to the hand when firing factory loads, so walnuts were made standard. I can't say there weren't a few hard rubber shipped on the earliest .44 flattops, because there may have been... but walnut was definitely standard.
By italian walnut, we mean that they had some nice figure in the panels. Keep in mind that Ruger offered walnut panels as an accessory before the first Single-Sixes were even shipped. The engraved Single-Six was also standard with walnut panels, with many of those showing tremendous burly grain. Since they had them on hand when the .44 flattop came it, it only makes sense they put the same walnut panels on their earliest flattops.
Chet15