O'Hara has signed the 'confession' for Grisby's fake murder believing that he can't be convicted because Grisby will have disappeared from view. No body, no conviction. They are ready to stage the murder but the event is bookmarked by two real murders.
Bannister's assistant Broome has overheard some of the plotting - concluding that Grisby is about to kill his boss - but when he threatens blackmail Grisby without hesitation pulls a gun and fires.
Then ... O'Hara, unaware of Broome's shooting, returns with Grisby to the Walhalla bar (at 201 Bridgeway in Sausalito, described previously here) and fires off a few shots to catch people's attention while Grisby slips away.
... and Now, the building is still there but has been widened and extended out closer to the boardwalk.
Then ... while O'Hara is attracting a startled crowd Grisby jumps unseen into a motor boat and heads past the anchored Circe towards the glittering San Francisco shore. Reader Wallace Berry has pointed out that this boat wasn't the Zaca (used for the Acapulco scenes), but a stand-in, the White Cloud.
... and Now, the same skyline, though far more developed, is as beautiful as ever - the Bay Bridge, as strings of pearls, only enhances the scene. A few skeletal remains are all that is left of the pier.
Then ... Grisby completes the crossing and arrives at a marina.
Then ... O'Hara phones Elsa only to reach the mortally wounded Broome whose gasping last words tell him Grisby is on his way to kill his law partner (Bannister), planning on pinning the murder on O'Hara. O'Hara frantically drives over the Golden Gate Bridge to Bannister's office in San Francisco.
... and Now, CitySleuth snapped this matching shot at the south end of the bridge from a location off limits to pedestrians (only cyclists can use this side) for which he was rebuked by the authorities. If you look carefully you can see the recent addition of a safety railing, elegantly done on each side, between the pedestrians/bikers and road traffic.
Then ... The camera pivots, following the car, and we see the Round House on the right, opened as a restaurant for passing motorists in 1938, one year after the bridge was inaugurated. In the distance just left of center is the dome of the Palace of Fine Arts and Coit Tower atop Telegraph Hill is discernible at far left.
... and Now, the Round House is still there, now a gift shop for the ubiquitous visitors.
Then ... O'Hara arrives at Bannister's office and finds the police are out front responding to yet another murder. But, surprise, it's Grisby who has been shot! The police find the confession on O'Hara and promptly arrest him.
Wouldn't you know it, this scene was filmed on the Columbia Ranch, the original back lot for Columbia Pictures, in Burbank (map). Below is an image from a 1949 Batman and Robin TV episode filmed at the ranch showing the same building, known as the Kirby Building. It was subsequently destroyed in a devastating 1974 fire but the ranch is still operational, now called the Warner Ranch.
Just to dot the i's, below is the Columbia Ranch layout as it was when the movie was filmed, with the Kirby Building corner shown arrowed. The two views above look across New York Street with Brownstone Street teeing in on the right. (CitySleuth is indebted to the excellent Columbia Ranch website for these images).