Tom,
You might have to use a wire brush to clean up the engine serial number. It is not a raised casting code; it is pressed into the ledge and might be filled with dirt/oil/etc.
On the other hand, it might be a factory replacement engine (I do not know how Chevrolet marked them); or, it might be a used replacement engine and the serial number has been ground off (I have a 1954 235 that has no serial number but I can see that the number has been ground off).
Tim
