When did Chrysler start stamp VINs on engine blocks?

I have a 340 block dated 06-22-67, so that should make it date correct for a 68 vehicle. I can’t find a VIN on it or even a stamping pad for the VIN. Did they start stamping the VIN on the engine blocks at the start of production of the 68 models, or at some point after they had started production?
The casting numbers are
0521 and 2780930-340-5
Thanks