There are several variations of marks on flywheels depending on the year and what engine it was originally on.
In this link I show some of these. Originally most engines were timed at the ball or circle, but a few were set at TDC, which is the line before or between UC.
The triangle is only for checking cam spec's.
With today's fuel I find most engines run better with about 5 degrees additional advance over stock timing.
