Rivera, now sure Howard Mason is the blackmailer, plots a way to dispatch him. He gets Sheila to invite him to her home at night on a pretense of signing a document. She will signal his departure to Rivera, waiting outside with a gun, in time for him to do the dirty deed. Although she detests the man, she turns on the charm when he arrives ...
Then ... From her upstairs window, Sheila watches Mason drive off. She opens the curtains and Rivera, at the top of the hill, sees the signal.
Now ... The only vantage point where Rivera could see the house would be the top of the block, at Baker and Pacific (map, blue marker, with the house at the red marker). This recent view from that junction looks to Sheila's home, marked by the arrow. This is the block the movie should have shown, but it didn't (... duh).
... and Then, instead, looking back with Rivera through his car's rear window we see Mason's car approaching up a totally different block. CitySleuth hasn't found it yet; fellow San Francisco sleuthers are invited to seek it out.
Rivera fires at Mason as he drives by but narrowly misses. The intended victim, badly shaken, returns to Sheila's place only to realize that she and Rivera were behind the attack. Enraged, he is about to strike her with a poker when Rivera arrives and fires again, this time right on target.