Back up plate washers are used with blind rivets for extra support when fastening into soft material. Zinc plated fasteners have good corrosion resistant qualities and are a better choice for indoor, outdoor and industrial applications.
Aluminum washers are corrosion and rust resistant.
If there is an "(S)" in the part name, it indicates that it has a smaller O.D, compared to the "(L)" which indicates that it has a larger O.D
For the product numbers that are sold by weight, 5lbs or 40lbs the individual pieces that are contained within the package are approximate due to manufacturers making some flat washers/lock washers slightly thicker, or thinner.