Hi Viking.

What bit don't you understand? Is it the actual direction of the arrows, or is it that it goes in one plug and out the other?

If it's the former, then I reckon the arrow direction is pretty academic - I think they are simply demonstrating that there's a complete 'loop' involving both plugs. (If you swapped the HT leads around, then it would go the other way, but would still work just the same.)

When the 'points' open, the magnetic field that was built up collapses instantly and this sends an electrical pulse back out the primary coil in the opposite direction to that it went in (I understand the main job of the 'condenser' is to absorb this large pulse and prevent it arcing a burning the points.) The much higher induced voltage in the secondary coil will also come out the opposite way to the pulse when the coil was switched on (points closing). (I think there may also be a 'spark' when the points close, but it's a relatively small one, and its timing means it has no effect on the ignition.)

So, the arrow direction is largely irrelevant - I think it's just to show what's happening. If you have something against that particular direction, say, for political reasons, then swap your HT leads or the primary coil wires

(There's also the small matter of actual current flow and 'conventional' current flow...)

I guess that should even out the wear on both plugs so's it occurs the same on both...

In theory it shouldn't matter which LT lead goes to which terminal on the coil, but I'd be tempted to keep it the way 'it was'!

What you could do, however, and I think it makes sense, is to swap around the spark plugs every time you take them out to check and adjust.