The 'ckx' could be one of these:
1972 CKX 350 tur hydro trans. K 200 4 Vette
1973 CKX 350 t/h, NB2 L65 145 2 Camaro
1976 CKX 350 a/t L L48 180 4 Vette
The other number should match up to the last part of your VIN. If is does, the 1976 code would make sense.
the 1203 part means the engine was assmebled on Dec 03. You have to check on the back side of the block near the casting number for the casting date code to verify the year.