Usually, the "BB" in the flywheel was the timing mark.
"Usually", because I think some years did not have a BB - just timing marks (one mark being TDC/UDC).
There were stovebolt six engines that timed at TDC.
Different years and different 216/235/261 engines could have timed before TDC, at TDC, or after TDC, depending on the year/equipment.
