I'm guessing these are GE transformers, manufactured in Oakland.

General Electric had a transformer plant in Oakland, CA starting in 1924.  (5441 East 14th/ International Blvd).

See "Project Background information" here.  
https://www.oaklandca.gov/documents/ge-site-remediation-and-redevelopment-project-draft-environmental-impact-report-deir-5441-international-blvd-case-file-number-er18013 

LA's 

Westinghouse Electric had a plant in Emeryville, CA from 1924, but it was apparently only building insulators, and later repaired transformers.
https://www.epa.gov/pcbs/pacific-southwest-pcb-success-stories-redevelopment-northern-california

Westinghouse did build transformers in Sunnyvale, CA, but only starting around 1946 when they bought Hendy Iron Works.  I did a quick search and couldn't find other mentions of Westinghouse manufacturing transformers in California.

https://cumulis.epa.gov/supercpad/cursites/csitinfo.cfm?id=0900956

Robert