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Oval countersunk heads used in few places where space is not available. Again since they are JIS specification the proper screwdriver is highly advised.
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JIS Screw Countersunk Oval Head
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 Widely used in the 70s it has been replaced by trivalent zinc because of its environmental hazard.
Chrome platting is used for a beautiful mirror finish. However it is used only for very visible bolts, never seen for screws. Acorn nuts should have this chrome platting for a perfect reflection. If you like a highly polished look this is the finish to use.
Stainless is not a plating but an alloy of steel made of iron and containing 12% Chromium in JIS screws. It is resistant to rust and corrosion so the aspect never changes. Disadvantages are mainly the price of stainless steel fasteners and the risk of galling when threaded into aluminum engine casing. A lubricant is highly recommended.