the 'date' code I think is back there but the block code is another thing... this is what posted on the 'ramblings' section whjere you had similar question..

I have a 72 block YX in my 69...
if your engine was repainted and is a 69 (Firebird Engine) it should have a two digit letter code on it under the valve covers on the front drivers side , like ,WC,WM,XL,XB,YJ,YE, WN,XC (350s),WZ,YT,WQ,YW,WH,XN (400s)for the different V8s

these numbers are all out of a Firebird redbook, there are other possiblities too if engine came out of another Ponitac