Classic/Antique Car Repair: 1951 chevy 216ci engine, good luck jack, chevy car

Question
why do some 216ci engins have oil filters and some dont.

Answer
I'm sorry, but I know Model A's and not too much about the modern iron.  In your case, I think the answer might be that the filter was an option.  That is right about the time they were going to full flow filter systems.  It also could have been a running change where the early built cars did not get a filter.  
Find yourself a Chevy car club and ask them.  Better yet, join the club and have more fun with your car.
Good luck.
jack