It may have been! I can tell you for certain that on my '67 NOR, the BB Z-bar was originally phosphate. I also have a NOS SB z-bar I recently found in its wrapper collecting dust in my shed and that has a greenish hue phosphate.
Today it would be considered gray but when it ages it turns towards a greenish hue like an old firearm. It looks like phosphate in your picture.
Mike