As Don notes, if you have approximate timing already you can dial it in by ear/seat of pants. Still, good to know where the mark is for initial set-up and if you get curious and want to look at advance with a timing light.
It is a hole in the back lip of the crank pulley that is at the top (12:00) when Number 1 &/or 4 are at top dead center.
If it helps, #1 TDC should occur when the dizzy rotor is pointing at #1 spark plug.
Of course it's possible that your mark is somewhere else or missing altogether but that's where is was when it left Coventry.