Metallics JPMP100BR Screw 1/4-20 x 3/4 PN PH MS BR (Jar 100)

It's a 1/4-20 x 3/4 inch pan head Phillips machine screw in brass. The 1/4-20 means a 1/4 inch diameter body with 20 threads per inch, and the 3/4 inch is the shank length under the head. Machine screw threads are straight the whole length, so it needs a tapped hole or a nut, not wood or drywall.

No. Boxes, covers and ground screws in standard US electrical work are 10-32 or 8-32. The JPMP100BR is 1/4-20, a coarser and larger thread, so it won't start in a 10-32 hole. Use it where the tapped hole or nut is actually 1/4-20, such as strut hardware, channel nuts, and larger equipment mounting points.

One hundred screws, packed in a jar. The jar pack is aimed at van stock and shop bins, so an electrician or maintenance tech can keep a single size on hand without loose bags coming open in a truck drawer.

Brass is used where you want a non-ferrous, non-magnetic fastener with better corrosion resistance than plain plated steel, and where you don't want galvanic trouble against brass or copper parts (grounding and bonding hardware is often brass). The tradeoff is strength: brass is softer than steel, so for high-torque structural connections a steel machine screw is the better pick.

Any 1/4-20 nut will thread onto it, since the screw is 1/4-20. Machine screws ship without nuts or washers, so order those separately. Brass or stainless nuts keep the joint non-ferrous; a plain steel nut on a brass screw works mechanically but gives up some of the corrosion benefit.

No. A pan head sits on top of the material with a low rounded crown, so it leaves a slight proud head rather than sitting flush. If you need flush, you want a flat head screw in a countersunk hole instead. The Phillips drive on the JPMP100BR takes a standard #2 or #3 Phillips bit depending on the recess.