Most road bikes have "slack wire retard" - if the cable breaks you can start the thing and get it home. Most racing bikes have "slack wire advance" - you can keep racing at full advance if the cable breaks.
Here's a perfect example from Bob Cordon Champ's Sunbeam book. In 1930 Sunbeams used square ML magnetos, anti-cw rotation for the drive end. On the road-going Models 8 and 9, the a/r cable is at the rear of the magneto, giving slack wire retard, while the racers Model 80 and 90 have the a/r cable at the front, giving slack wire advance.
I don't think I've ever seen a "slack wire advance" Lucas magdyno, but if there was such a thing you might find it on a road-going 90 Plus. Because the bikes we play with are so old, you need to check what is currently fitted to your bike.
Cheers
Leon