I'm not certain, but I believe the change to the valve cover oil fill was related to emissions controls, and the pcv valve. Previously there was an opening in the back of the block behind the distributor and the crankcase was vented from this opening via a fitting and rubber hose. The opening was eliminated when the pcv valve setup instituted. So a 69 block does not have the opening. I'm not sure about 68, but I had a 67 327 which had the opening and was vented out the rear of the block. It did not have a pcv valve, and there were no openings in the valve covers.