On December 8th 1941, the United States officially entered World War II after the surprise attack on Pearl Harbor the previous day. Fearing subsequent attacks, the city of Los Angeles (among other major cities) installed a series of air raid sirens scattered throughout the city that could provide an early warning system in such an event.
After the war ended in 1945, there wasn’t much use for these sirens until Cold War tensions with Russia once again created the fear of invasion, though this time the stakes were far higher due to nuclear weapons being on the table. Other cities began adopting similar systems in the following decades, most of which were later adapted into early-warning systems for tornadoes and other natural disasters.
The siren network in Los Angeles was maintained and regularly tested until officials decided to decommission the entire system in 1985, leaving the sirens to rest forever as relics of Los Angeles history. There were initially 226 sirens in place around Los Angeles, and despite not being functional any longer, about 60% of them are still around. If you'd like to learn more, there's a really great (and really old) database at wirechief.com.
The siren at this location is right across the street from the cache, to the west. It's a Federal Signal model SD-10, nicknamed "wire spool."
Congrats to setdecbeth for FTF!