“Pom-Pom” was the name given to the 37mm Maxim gun by the Boers of South Africa, based on the gun’s sound. It was a Maxim machine gun scaled up to the quite impressive 37mm caliber, intended primarily for naval use defending large vessels against small torpedo boats. This particular example is serial number 2024, made in 1889 and then sold three times before being ultimately purchased by the United States Coast Guard and installed on the USS Manning (along with a second gun, number 2026). The Manning was promptly put into military service by the Navy and steamed down to Cuba, where it participated in the first bombardment of Cuba during the Spanish-American War.
These 37mm guns could fire a wide variety of projectiles, including solid rounds which could pierce an inch (25mm) of iron armor at 100 yards and hollow rounds filled with black powder and fused to explode on impact. During World War One, they would be pushed into anti-aircraft service, with the explosive rounds being extremely effective on early aircraft (when you could get a hit, anyway).