Checking the index of Motor Coach Companies by state, the only one with the name "Motor Transit Co" is the only one listed. Here's the record Feb. 75 is the date it was formed. Known as a "stage" company, the firm operated bus service to multiple cities in California. Similar to Union Pacific Stage Co. -- that provided service between sections of railroad stations that did not have connecting rail service -- which was many in early development of rail service throughout California. Here's a link to the index:

http://www.motorbussociety.org/mca/MCA-MCT%20Index%201950-2006.pdf

Here you will find Motor Coach Age, a magazine first published in 1950 and continually updated ever since. There is also an association that has a physical library and all the archives of Motor Coaches. Here's that link that gets you to the archives. You can submit your plate and get a more definitive identification, but they ask for a donation to do that for you.

http://www.motorbussociety.org/

The illustration below is the U.S. Index and yours is listed under California" Your brass plate is the cab identification plate found inside every bus. You can find out the age based on the number. Similar to a herald on early railroads that are displayed with company name and number.
Hope this helps.