I recently saw a similar Cadillac as a staff car, unrestored, in a private collection in Allentown, PA

http://tinyurl.com/px9un78

There is apparently only a single surviving WWI Cadillac, according to this article.

There were actually three identical balloon hangars in the SF area during the 1920s: Fort Winfield Scott (inside the Presidio), Fort Barry, and Fort Funston. The Shorpy view shows the Fort Funston hangar, the giveaway being the tree line in the background. There's no landscape like that at either the Presidio or Fort Barry.

Here's a photo from a National Archives series showing the same searchlight exercises, but this time the searchlights have been dismounted. The Shorpy truck might even be one of the two in this view.

The searchlight is riding on Ford Model T front axles, front and rear. I can't tell if the original steering parts are operational, or if it has center-pivot steering, Radio-Flyer style. Looks like a bespoke "chassis" to go along with the bespoked wheels.

This is probably Crissy Field, given the hanger hangar in the background. It's a lovely park now, but back in the day it was where the Army kept their airplanes. It's a a bit farther inside the bay than the Presidio, and pleasantly flat.

[The building is the Fort Barry balloon hangar, on the side of a hill. -Dave]

The U.S. Army held competitive tests during the summer of 1917 in Marfa, Texas to choose a standard staff car, and Cadillac was selected as the winner. Eventually thousands of Cadillac Type 55 eight-cylinder cars went to the front. The English, Canadians and French had already been using Cadillacs as staff cars and ambulances.

The Cadillac shown here had a special 145-inch wheelbase, and used an engine-driven generator to power the searchlight. This appears to be a 1917 model that probably started out as a Type 55. The hoods of the military models were different with many more louvers than the standard models. A top speed of 50 mph was possible fully loaded with the searchlight stowed.

The searchlight was manufactured by the Sperry Company, using an open system that emitted 600 million candlepower at 78 volts and 150 amperes. The light was five feet in diameter, and it rolled off of the back using ramps. The range of the light was 15 miles.

One of the photos below has the same identification number on the hood as the Shorpy photo, and in the other you can see the winch lines.

Whitewall tires.

Considering that this is at the Presidio, this might be a Coast Artillery Corps mobile light, rather than an anti-aircraft light. I can't make out any insignia on their uniforms, but someone with better photographic processing skills might be able to. The hand wheel adjusts the field excitation of the generator, which is driven by a power take-off from the transmission.

This looks like an early version of the weapon they used to bring down the Saucers.