[v8volvo]

Mine have always had a "0" in the correct location, but it can be hard to see.

Did your cam timing belt slip too, or just the rear pump belt? If the front belt is still intact, you can use the cam to help you figure out where TDC is, assuming it was timed more or less correctly last time someone put a belt on it.

I've never seen one where the 0 was incorrect. There is only one way to put the flywheel on, so it would be hard for it to be wrong. If it were me, I would try timing it both ways (to the 0 and then to the line), and spin the motor over by hand with a wrench and see which position makes the valves and pistons collide. Either way if you are unsure you will want to give the motor plenty of rotations by hand before you ever try spinning it with the starter -- you don't want to risk busting up the valves!

Make sure you're able to see everything clearly on the flywheel -- sometimes dust and corrosion can obscure the markings. Sometimes I stick a wire brush down in the hole and then spin the crank so that the surface gets cleaned up a bit, and then you can additionally use a paint pen on the marking locations to make them clearer.