From a great blog post on the film The Big Sleep: http://www.waynesthisandthat.com/bigsleep.htm
Agnes Lowzier, the saleslady at Geiger’s bookstore and Joe Brody’s accomplice, played by Sonia Darrin. Although her face is very familiar it’s difficult to find information on her. She’s appeared in only seven films, all but one of which were uncredited. It’s odd she was uncredited in The Big Sleep because she had as much screen time and dialog as Carmen.
In 2016 Ms. Alice Griffin very kindly contacted me to provide a link to an article with an interview with Sonia Darrin explaining why she didn’t receive screen credits. After filming but before the first release, Ms. Darrin’s agent got into a violent argument with studio head Jack Warner. As a consequence, Mr. Warner declared he would never hire anyone connected with the agent and had Ms. Darrin’s name stricken from the screen credits. He might have gone as far as cutting her from the movie altogether, but her scenes were too important to do so. Ms. Darrin’s career never took off after that and she moved to New York, where she was successful as a top model. I’d like to express my profound appreciation to Alice Griffin for resolving this mystery. (Thanks, Alice!)