During World War I, the British Army utilized a variety of artillery pieces and corresponding ammunition, including the 3-inch 20cwt (hundredweight) shell. The 3-inch 20cwt shell case was used in artillery with a caliber of approximately 76.2 mm. Here's a general description of such a shell case:

1. Caliber: The "3-inch" designation refers to the approximate diameter of the shell, which is around 76.2 mm.

2. Weight: The "20cwt" indicates the weight of the gun. In this case, it's a 20 hundredweight gun, equivalent to 1,016 kilograms. This classification is a measure of the gun's loading weight, not the weight of the projectile itself.

3. Material: The shell case was typically made of brass or another non-ferrous metal. Brass was a common choice due to its resistance to corrosion and relatively easy manufacturing process.

4. Markings: The shell case would likely have various markings indicating its origin, production date, and possibly the manufacturer. These markings often include information such as the broad arrow symbol, the crown, and letters or numbers representing the manufacturer and production date.

5. Base: The base of the shell case might have additional markings, and it would typically feature a primer pocket where the percussion primer was inserted.

6. Purpose: The 3-inch 20cwt shell was used in various artillery pieces during World War I, including field guns and anti-aircraft guns. It was employed for a range of purposes, such as direct fire against enemy positions or indirect fire to provide support for infantry.