The A 20 9 head date-casting code is probably, January 20th 1959.
Here are locations of various head and block codes. A Rochester carburetor was the standard carburetor from about 1950 through the end of the run of the 235/261 engines.
If there is no engine serial number and that number/code has not been ground off, it is possible you have a replacement block.
Sometimes the vehicle serial number was used for titling/registration and sometimes the engine serial number was used for registration/titling.
