Read my first post....I didn't say the timing indicator is missing. It's there, where it was factory installed. I said I doubt the factory made a mistake but, I was spit-balling because it was a highly transitional time for the S30.
Anyway, I got my answer. A friend of mine was a Datsun tech in the 70s and later a Nissan master tech. He said the dealer installed AC kit came with a 2-row pulley to replace the non-AC 2-row pulley, which had a narrower groove since it only drove the air pump. Do you see where this is going? The kit supplied pulley (the one I have) was the same as the factory AC pulley, meaning the timing mark would now be on the right side of the motor. That also meant the timing indicator needed to be swapped to the right side as well. He said these installs were hard to hack, but the $5/hour installers would sometimes leave off the final step of moving the timing indicator to the other side since a timing check was not a part of the AC install.
Satisfies my curiosity! It runs well and I'll leave everything as is and make a new timing mark in the same location as a non-AC pulley when I have the valve cover off....maybe this fall. Or put a timing indicator on the passenger side.
Right now I'm just enjoying driving this survivor time machine with it's cold AC, oh, and the sound of the nos Pacesetter vertical tips I just installed....reminds me of the 70s.