High strength steel nuts are now available. Most options are covered; standard or flat, different pitches standard (coarse) or fine, one bevel or two sided bevel, and standard or small head (refer to the drawing for sizing).
In the Japanese industrial standard (JIS B 1181), S45C nuts are typically manufactured to "Strength Class 6" or "8" (equivalent to the 10.9T bolts)
S45C: This is a Medium Carbon Steel (JIS G4051). The "45" indicates a carbon content of approximately 0.45%.
-H: The "H" typically stands for Hardenability Guaranteed (often referred to as "H-band" steel).
S45C-H High Strength Nut
Zinc is now called trivalent plating treatment which is a modern plating that looks similar to zinc without the chemical hazard and better consistency and corrosion resistance. This is the standard for Japanese manufacture and I recommend it for a perfect restoration.
Black aspect trivalent plating treatment which is a modern plating that looks similar to black zinc without the chemical hazard and better consistency and corrosion resistance. This replaces black zinc of early models and is now standard for Japanese manufacturers.
Yellow chromate treatment is a plating often found on small screws for carburetor or to give a rich aspect. Widely used in the 70s it has been replaced by trivalent zinc because of its environmental hazard.