No -- it must be notarized and apostilled -- but of course the sig of the SS person can't be notarized because he/she is not present. That's why I signed the letter and had my sig notarized -- which is meaningless, but it satisfied them.
Then they will give you is a 'certification' of income, which summarizes (in Spanish) the info on the SS letter.
For your circumstances, you need to get the SS letter notarized and apostilled before you go to LA.